Hi, today I will introduce you a docker functionnality I discovered 6 months ago: docker contexts 🐳.

It allows you to build and run your containers on a distant host.

Please note I'm only a docker user, for further informations about contexts visit the official documentation

🧠 Understand contexts

Actually, docker contexts let you manage multiple docker daemons from a single client.

A context has two major informations:

Its name and description
Its endpoint

By default, a default context is created and used. Its endpoint is the local docker sock.

NAME         DESCRIPTION                               DOCKER ENDPOINT               ERROR
default                                                unix:///var/run/docker.sock

The strength of contexts is that you can change from one host to another in only one command !

⚡ Get started with contexts

⚙️ Sever configuration

First, you need to configure your server to enable remote access to your docker daemon.

⚠️ Remote access permit anyone in a network to connect to your docker daemon and, potentially have root access to your server ⚠️

See official documentation

Two methods to enable remote access:

With systemd
With daemons.json

With systemd

1.) Edit the docker.service file and put your own values.

sudo systemctl edit docker.service

[Service]
ExecStart=/usr/bin/dockerd -H fd:// -H tcp://0.0.0.0:2375

2.) Reload changes and restart docker daemon

sudo systemctl daemon-reload && sudo systemctl restart docker.service

With daemons.json

1.) Add the folowing to /etc/docker/daemons.json

{
  "hosts": ["unix:///var/run/docker.sock", "tcp://0.0.0.0:2375"]
}

2.) Restart docker

You should now have remote access enabled. You can verify docker is binding on port 2375 using netstat:

$ sudo netstat -lntp | grep dockerd
tcp        0      0 127.0.0.1:2375          0.0.0.0:*               LISTEN      3758/dockerd

💻 Client configuration & examples

First, you need to ensure that your docker cli supports the context command:

docker context

It should show you a help about contexts

Next, let's create a context

docker context create <context> --docker host=tcp://host:2375

Replace <context> with its name and host by the docker daemons IP.

It should be in the list of contexts outputted by

docker context ls

NAME         DESCRIPTION                               DOCKER ENDPOINT               ERROR
default *                                              unix:///var/run/docker.sock   
<context>                                              tcp://<context-ip>:2375

Default as an asterisk after its name (default *) because it's the currently used context.

To change to the newly created one, just type.

docker context use <context-name>

Now you can run any docker container in the distant docker daemon

docker run -d -it busybox /bin/sh

This execute a shell in an empty container

You should see it in docker ps

docker ps

CONTAINER ID   IMAGE               COMMAND                CREATED         STATUS                          PORTS     NAMES
f4d3674af80a   busybox             "/bin/sh"              2 seconds ago   Up 1 second                               focused_ramanujan

You can verify that it's been running on the distant host by passing which context to use directly in the command:

docker --context default ps

Should not output busybox container
Whereas docker --context <context> ps should !

You have know the rudiments about docker context. Consider referring to the Docker documentation for further informations and explanations.

📈 How I use it ?

Discovering this feature was very helpful in my workflow: I don't need to use ssh to deploy latest versions of my apps anymore.

I just change context, to choose the right server (like databases, microservices, web-prod) and fire docker run -d ....

In parallel i use Portainer, a powerful web UI for docker. It allows me to manage my running containers and check their health right in my browser !

Here are some screenshots of my portainer: