In this article, we embark on an insightful journey into the realm of Docker essentials, unraveling all the knowledge needed to grasp this groundbreaking technology. This involves delving deep into docker commands with examples and docker compose as well. Lets dig in!!

Docker definition

Docker can be defined as an application used to automate the deployment, management and scaling of applications. Its runs applications in an isolated environment very similar to virtual machines but slightly better. Docker works with "boxes" called containers which as the name suggests, holds everything that is required to run the application, from the code to its dependencies. Multiple containers can run on the same machine without affecting each other, since each container has its own dependencies. They all however share the same kernel(acts as an intermediary between the hardware and the software, providing essential services and functionalities to manage system resources, facilitate communication between hardware and software, and ensure overall system stability and security.)

Advantages of docker

1. It uses less memory space, since the application are stored in the docker container and not locally.
2. They are first to boot with just one command once the container is set up.
3. They are easy to set up compared to locally running instances of the same application which are limited to the running computer's capabilities and easy to scale when needed.
4. Easy to share with others, deploy and test applications.

The installation of docker and docker-compose(discussed later in the article) are limited to the type of OS a user is running. You can visit this link for installation guidance.
To verify if docker is correctly installed use the following command:

docker --version

Output example depending on the version installed
Docker version 24.0.4

Docker Image

This is a package template used to create one or more containers as we shall see in the examples bellow.

Docker hub

There are some common tool used in day to day programming like languages eg python, web servers eg nginx and databases eg postgresql , which are used when creating various applications. These tools are stored in a registry called docker hub in form of images. You need to create an account in order to access dockerhub and utilize its images.
Once the account is created, we can easily pull images from docker hub.
The images you create can also be pushed into docker hub and made public or private.
The following command pulls a python version 3.10 image from docker hub

docker pull python:3.10-alpine3.18

The alpine3.18 keyword pulls the smallest in size of python3.10 available , hence saving on storage space. It is advisable to use "alpine" when pulling or defining images.

To view all the images we can run

docker images
Output:
REPOSITORY       TAG               IMAGE ID       CREATED       SIZE
python           3.10-alpine3.18   7d34b8225fda   7 weeks ago   49.7MB

Running a normal installation of python3 using docker pull python:3.10 then viewing installed images, displays the difference in size between the normal one and the alpine version as shown:

REPOSITORY       TAG               IMAGE ID       CREATED       SIZE
python           3.10              d9122363988f   6 weeks ago   1GB
python           3.10-alpine3.18   7d34b8225fda   7 weeks ago   49.7MB

To remove an image run

docker image rm <image_name/image_id>

Example:
docker image rm python:3.10

Output:
d9122363988f

Docker Container

Once the image is defined, it can be used to create a container, since a container runs an instance of an image.
For example using the python image above:

docker run --name pytry -it python:3.10-alpine3.18

The --name flag is used to define the name that will be used to reference the container, else a random name will be given to your container.
The -it command is used to run the command in an interactive mode.
The above command produces a python shell which we can use to run various commands as shown.

Python 3.10.12 (main, Jun 15 2023, 03:17:49) [GCC 12.2.1 20220924] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> print("I am a python container")
I am a python container
>>> 

To view all the flags that can be used with the docker run command use:

docker run --help

Now that the container is defined, run the command bellow on another terminal window to view running containers

Command:
docker ps

Output:
CONTAINER ID   IMAGE                    COMMAND     CREATED          STATUS          PORTS     NAMES
5bcde232650b   python:3.10-alpine3.18   "python3"   32 seconds ago   Up 31 seconds             pytry

To view all the container which are present in docker use:

docker ps -a
Move to the terminal window running python container and run the command exit() to return to the command line.
Notice running docker ps to view running containers produces no results since our container is closed.

To make the code above more complex we can create a folder called hello.py in the current directory and write some code in it. Then run the command again but with a different container name to avoid conflicts.

# file in the current directory
hello.py
from datetime import datetime
time = datetime.now()
print(f'I am a python file in a docker container running at {time}')

Command:
docker run --name pytryFile -w /app -v "$(pwd):/app" python:3.10-alpine3.18 python hello.py

Output:
I am a python file in a docker container running at <a_timestamp_of _the_time_right_now>

Let's break down this command:

• -v "$(pwd):/app" mounts the current directory (pwd) into the container at the /app directory, allowing access to your Python script.
• -w /app sets the working directory inside the container to /app.
• python:3.10-alpine3.18 is the image name
• python hello.py is the command that runs your Python script inside the container.

Managing Containers

To stop a running docker container use:

docker stop <container_name/container_id>
example:
docker stop pytry

Output:
pytry

To start a stopped container:

docker start <container_name/container_id>
example:
docker start pytry
Output:
pytry

To remove a docker container :

docker rm <container_name/container_id>
example:
docker rm pytry

To list all container's id

docker ps -aq

To get rid of all of the containers:

docker rm $(docker ps -aq)

To view other interesting flags you can use to manipulate containers:
docker ps --help

Docker Volumes

They enable easy sharing of information like files and folders between host and container and between containers themselves, just as the hello.py example above demonstrated.
Another example, we create a new folder and inside it , a file called hello.html that displays:

<h2> Hello World </>

To display this content on a web browser using the nginx web server, run the nginx image as shown below:

docker run --name helloWorld -v $(pwd)/hello.html:/usr/share/nginx/html/hello.html:ro -p 8080:80 -d nginx

# View the content on the browser as shown:
http://localhost:8080/hello.html

/usr/share/nginx/html: is the nginx folder used to display content stored in nginx web server
ro: includes permissions on the shared content, in our case read and write permission.
-p: this flag illustrates the port mapping in which the contents will be viewed from. In our case all the contents on the container(80) will be viewed on host by accessing port(8080)

Note: we did not pull the nginx image before using it , but running "docker images" after the above command is successful returns an nginx image as one of the images present. This is because when executing docker run ..., docker checks for the presence of the image locally and once it can't find it, it pulls it from docker hub and runs it

docker images
Output:
REPOSITORY       TAG               IMAGE ID       CREATED       SIZE
python           3.10-alpine3.18   7d34b8225fda   7 weeks ago   49.7MB
nginx            latest            021283c8eb95   3 weeks ago   187MB

# running containers
docker ps

Output:
CONTAINER ID   IMAGE     COMMAND                  CREATED          STATUS          PORTS                                   NAMES
eefaa7cd973c   nginx     "/docker-entrypoint.…"   11 minutes ago   Up 11 minutes   0.0.0.0:8080->80/tcp, :::8080->80/tcp   helloWorld

Dockerfile

This is a file given the name "Dockerfile" in running applications and is usually used to build applications which contain multiple parts and share them.
In simple terms a docker file is used to make our own personal images.
It contains a list of steps used to bring the app to life.
A docker file has its own structure, always starting with a FROM clause and the rest of the flags can come interchangeably depending on the requirements of the application.
The FROM clause can also appear multiple times to create multiple images.
Example of commands that can come after the FROM clause include:

FROM - statement defining the image to pull
RUN - to execute commands
WORKDIR - define the directory to work from
COPY - create a duplicate o the files or folders present in the host to the container.
ADD - copies new files, folders to the remote directory.
RUN - execute a command
CMD - run a file

More commands like this one can be found in the docker reference

An example of a dockerfile includes:
Assuming we have a folder called Example,with a hello.js file
containing

console.log('hello, world');

Create another file and name it "dockerfile" with the content below:

FROM node:alpine
WORKDIR /app
COPY . .
CMD node hello.js

Docker Build

Once a docker file is defined, the "build" command is used to create the image. View docker build --help to view the flags that can be used with docker build.
The syntax is as shown:

docker build --tag <name_of_image>:<tag> <directory_with_dockerfile>

The tag represents the image versioning which aids in image naming. If none is given, then latest is assumed.It provides more control of the image you create.

Creating the nodejs image above

docker build --tag greetings:latest .

Viewing the images as shown above, we notice a new image with a tag greetings:latest

REPOSITORY       TAG               IMAGE ID       CREATED       SIZE
python           3.10-alpine3.18   7d34b8225fda   7 weeks ago   49.7MB
nginx            latest            021283c8eb95   3 weeks ago   187MB
greetings           latest            531620bb45c5   17 seconds ago   181MB

Running the image:

docker run greetings:latest

Output:
hello, world

.dockerignore

It is used to ignore files that the application does not require to run like node_modules, requirement.txt and .git.
It is created as file in the current directory and the files to be ignored placed inside it

.dockerignore
requirement.txt
node_modules

Docker Registry

As discussed before images are pulled and pushed from docker hub. These images are stored in a repository called docker registry.
That said it is possible to push personal images to docker hub.

1. First create an account in docker hub.
2. Run the following command and provide your Docker Hub credentials (username and password) when prompted: docker login
3. Tag the previously built Docker image with your Docker Hub username and the desired repository name. docker tag greetings your_username/greetings:latest
4. Finally, push the tagged image to Docker Hub using the following command: docker push your_username/greetings:latest This will push the Docker image to your Docker Hub repository named "greetings" with the "latest" tag.

Docker Inspect

This command allows you to retrieve detailed information about Docker objects, such as containers, images, volumes, and networks. It provides a JSON representation of the specified Docker object, which includes various metadata and configuration details.
The syntax is as shown:
docker inspect <container_name /id >
Pick a container and try it out.

Docker Logs

This command is used to monitor traffic of a running container.
The syntax is as shown:
docker logs <container_name /id >
To follow the logs of the container as they come in add the -f flag before the container name/id.
To add a timestamp to each log when following add the the flags -ft before the container name/id.
For more important flags view
docker logs --help

Docker Network

This command is used to place two or more networks on the same network hence making it easy for them to communicate with each other. For example a postgres container and a pgadmin container that provides a graphical user interface of the data in a postgres database can easily communicate if they are placed on the same network.
The syntax is as shown:
docker network create <network_name>

The two images will have to now contain the flag --network when run in order to communicate eg
docker -e POSTGRES_PASSWORD=pass -p 5432:5432 --network <network_name> -d postgres:latest

Docker Compose

It allows you to define and manage multiple docker containers as one instead of linking them using a network.
This command is used to bring multiple parts of an application,eg frontend, backend and storage to life with just one command.
This is especially useful for development and testing environments, as well as for deploying applications to production.
It is mainly done using a docker-compose.yaml file containing services, networks, and volumes. Each service represents a container, and you can specify their images, ports, volumes, environment variables, etc.
The YAML file should be placed in the root directory of your project.
Ensure you install docker-compose according to your os type before usage. The docker-compose version should also be compatible with your docker version.
An example of a docker-compose file includes:

services:
  db:
    image: postgres:15.3-alpine3.18
    environment:
      - POSTGRES_PASSWORD=<password>
      - POSTGRES_DB=<db_name>
    volumes:
      - "./green:/var/lib/postgresql/data:rw"
    ports:
      - "5432:5432"
  admin:
     image: dpage/pgadmin4
     environment:
        - PGADMIN_DEFAULT_EMAIL=<any@email.com>
        - PGADMIN_DEFAULT_PASSWORD=<pgadmin_password>
     ports:
        - "8080:80"
     volumes:
        - "./pgadmindata:/var/lib/pgadmin:rw"

You can include the version of docker compose you are using eg version:3.8, just above everything else but its not a requirement by default the installed version will be used.

Let's unpack the file above:

• A service: contains various parts of your application. It also directs docker on how to build the image required by that service. Each service has to have a unique name that differentiates it from another. We use db as the service title for our postgres database image and admin to reffere to the pgadmin image and the flags it will use to run successfully.

To run the file :
docker-compose up

Use docker ps to view if the containers were created successfully and are up and running.
So generally our docker-compose file runs both postgres and pgadmin which is postgres gui in the same network.

Running https://localhost 8080 provide a pgadmin interface where you can log in using pgadmin credentials and create a server to view the postgres database with on the browser as shown.

Create a server name

Move to connection and use the name of the postgres service as the hostname and include the postgres user and password in the columns indicated

Now you can run

docker exec -it postgres:15.3-alpine3.18
psql -U <postgres_user>
\c <db_name>
CREATE TABLE hello_pdadmin;

Refresh pgadmin to see the changes.
To build the images and not run them, use
docker-compose build
To stop the running containers use
docker compose down

Conclusion

In conclusion, Docker is not just a technology but a mindset that encourages continuous integration and continuous deployment. Whether you're an individual developer, part of a team, or managing large-scale infrastructures, Docker has something to offer for everyone.Embracing Docker's capabilities will undoubtedly enhance your development workflow and bring your projects to new heights of efficiency and reliability. Thank you for staying curious and happy hacking!!