.
├── bin # helper docker-compose scripts
│   ├── build
│   ├── go
│   └── start
├── dev-ops
│   ├── dev.Dockerfile
│   ├── docker-compose.yml
│   └── prod.Dockerfile
├── README.md
└── source # go source files
    ├── crop.go
    ├── delivery.go
    ├── go.mod
    ├── go.sum
    ├── main.go
    └── vendor # app's go dependencies source files

## ./dev-ops/dev.Dockerfile

FROM golang:1.15-alpine3.12

## Make app dir for source files
## Get git binary so that GO can download dependencies
RUN mkdir /app \
    && apk add git

## Set app folder as work directory (default context in Docker container)
WORKDIR /app

## get and install watcher
RUN go get github.com/canthefason/go-watcher \
    && go install github.com/canthefason/go-watcher/cmd/watcher

## Turn on Go modules, it allows easier dependency managment
## Copy files with modules (dependencies) requirements, so that GO knows what to download
## Reminds me package.json and yarn.lock in JS development
ENV GO111MODULE=on
ADD ./source/go.mod /app
ADD ./source/go.sum /app

## pull in all modules (dependencies)
RUN go mod download

## Add source files to image work directory
ADD ./source /app

## Command which applies when container from this image runs
CMD ["watcher"]

## ./dev-ops/docker-compose.yml

version: '3'
services:
  ## ids is image delivery service (the service I wrote)
  ids:
    ## name of our image built from dev.Dockerfile above
    image: registry.gitlab.com/dashers/image-delivery-service/ids:dev
    container_name: dasher_ids
    ## sync your local source files with those in container
    volumes:
      - ../source/:/app/
...

## ./dev-ops/prod.Dockerfile

FROM golang:1.15-alpine3.12

RUN apk add git

WORKDIR /go/src/gitlab.com/dashers/image-delivery-service

## same as in dev... add files with dependencies requierments
ENV GO111MODULE=on
ADD ./go.mod .
ADD ./go.sum .

## pull in any dependencies
RUN go mod download

## add source files
ADD . .

## our app will now successfully build with the necessary go dependencies included
## creates ./main binary executable file
RUN go build -o main .

## runs our newly created binary executable
CMD ["./main"]

## ./gitlab-ci.yml

image: docker:stable

variables:
  # SET DEFAULT BEHAVIOR FOR CI
  # Disable submodules on CI, we are not using submodules
  GIT_SUBMODULE_STRATEGY: none
# important! we need to say gitlab that we run docker container in docker container..
services:
  - docker:dind

stages:
  - BUILD and PUBLISH

## template for build and publish stage
.build-and-publish_template: &build-and-publish_template
  stage: BUILD and PUBLISH
  ## I version project using commit tags. 
  ## Here I say I want to trigger the stage only if commit tag has format ids-x.x.x
  rules:
    - if: $CI_COMMIT_TAG =~ /^(ids)-[0-9]+\.[0-9]+\.[0-9]+$/
      when: always
    - when: never    
  before_script:
    # accept ids-0.0.1 => 0.0.1 | master => master
    - VERSION=$(if [ "$CI_COMMIT_TAG" == "" ]; then echo $CI_COMMIT_REF_NAME; else echo $CI_COMMIT_TAG |awk -F- '{print $2}'; fi)
    - echo $VERSION
        # login to gitlab container registry
    - echo -n $CI_REGISTRY_PASSWORD | docker login --username $CI_REGISTRY_USER --password-stdin $CI_REGISTRY
  after_script:
    # clean up
    - docker logout $CI_REGISTRY

## build and publish stage (using the template above)
ids - build and publish: 
  <<: *build-and-publish_template
  script:
    # build our production image (prod.Dockerfile)
    # registry.gitlab.com/dashers/image-delivery-service/ids:x.x.x
    - >
      docker build \
        --network host \
        --tag ${CI_REGISTRY}/dashers/image-delivery-service/ids:${VERSION} \
        --file ${CI_PROJECT_DIR}/dev-ops/prod.Dockerfile \
        --rm \
        ${CI_PROJECT_DIR}/source

    # publish into docker registry
    - docker push ${CI_REGISTRY}/dashers/image-delivery-service/ids:${VERSION}