CI/CD Pipeline config for GitLab to publish ASP.NET Core 6 web app in docker container

I need to start by saying that I am a beginner with CI/CD and with Gitlab. That being said, I have managed to cobble together (using examples from the web) this "almost" working pipeline configuration:

image: mcr.microsoft.com/dotnet/sdk:6.0

variables:
  OBJECTS_DIRECTORY: 'obj'
  NUGET_PACKAGES_DIRECTORY: '.nuget'
  SOURCE_CODE_PATH: '*/*/'
  TAG_LATEST: $CI_REGISTRY_IMAGE/$CI_COMMIT_REF_NAME:latest
  TAG_COMMIT: $CI_REGISTRY_IMAGE/$CI_COMMIT_REF_NAME:$CI_COMMIT_SHORT_SHA

stages:
  - build
  - test
  - docker
  - deploy

cache:
  key: "$CI_JOB_STAGE-$CI_COMMIT_REF_SLUG"
  paths:
    - '$SOURCE_CODE_PATH$OBJECTS_DIRECTORY/project.assets.json'
    - '$SOURCE_CODE_PATH$OBJECTS_DIRECTORY/*.csproj.nuget.*'
    - '$NUGET_PACKAGES_DIRECTORY'
  policy: pull-push

before_script:
  - 'dotnet restore --packages $NUGET_PACKAGES_DIRECTORY'

build:
  stage: build
  script:
  - 'dotnet build --no-restore'

tests:
  stage: test
  script:
    - 'dotnet test --no-restore'

docker:
  stage: docker
  image: docker:latest
  services:
    - docker:18.09.7-dind
  variables:
    DOCKER_DRIVER: overlay2
    DOCKER_TLS_CERTDIR: ""
  only:
    - main
before_script:
    - apt-get update
    - apt-get install docker -y
  script:
    - docker login -u "gitlab-ci-token" -p "$CI_JOB_TOKEN" $CI_REGISTRY
    - docker build --tag "$CI_REGISTRY_IMAGE:$CI_COMMIT_SHORT_SHA" --tag "$CI_REGISTRY_IMAGE:latest" .
    - docker push "$CI_REGISTRY_IMAGE:$CI_COMMIT_SHORT_SHA"
    - docker push "$CI_REGISTRY_IMAGE:latest"

deploy:
  stage: deploy
  image: alpine:latest
  services:
  - docker:dind

  script:
  - chmod og= $ID_RSA
  - apk update && apk add openssh-client
  - ssh -i $ID_RSA -o StrictHostKeyChecking=no $SERVER_USER@$SERVER_IP "docker login -u gitlab-ci-token -p $CI_BUILD_TOKEN $CI_REGISTRY"
  - ssh -i $ID_RSA -o StrictHostKeyChecking=no $SERVER_USER@$SERVER_IP "docker pull $TAG_COMMIT"
  - ssh -i $ID_RSA -o StrictHostKeyChecking=no $SERVER_USER@$SERVER_IP "docker container rm -f atlas-portal-2.0 || true"
  - ssh -i $ID_RSA -o StrictHostKeyChecking=no $SERVER_USER@$SERVER_IP "docker run -d -p 80:80 --name atlas-portal-2.0 $TAG_COMMIT"
  environment: 
    name: production
  only:
  - main

When I run this pipeline, it successfully gets to the “docker” stage where I hope to build and store the docker image in the gitlab container registry. Instead I get the following error:

> "/bin/bash: line 149: docker: command not found"

I know that somehow I need to “install” docker in the docker image that is going to be building the docker image to store in gitlab, but I am not sure how to go about doing that.

And once I get past this “docker” stage I feel confident that the “deploy” stage is going to fail too. I am a newbie...

Your docker job should look like this instead:

docker:
  stage: docker
  image: docker:latest
  services:
    - docker:dind
  variables:
    DOCKER_DRIVER: overlay2
    DOCKER_HOST: tcp://docker:2375
  only:
    - main
before_script:
    - docker login -u "gitlab-ci-token" -p "$CI_JOB_TOKEN" $CI_REGISTRY
  script:
    - docker build --tag "$CI_REGISTRY_IMAGE:$CI_COMMIT_SHORT_SHA" --tag "$CI_REGISTRY_IMAGE:latest" .
    - docker push "$CI_REGISTRY_IMAGE:$CI_COMMIT_SHORT_SHA"
    - docker push "$CI_REGISTRY_IMAGE:latest"

As you're already using an image that has docker installed, there is no need to run apt-get update && apt-get install docker -y, just run the command and that will make up for it.

Also another piece of advice, it'd be best for you to define the image value for each job unless you will use the same for all of them. So the one you have defined at the top of the file to use dotnet would be better placed at build and tests jobs. If you don't want to repeat code you can use variables to reuse the value, e.g:

variables:
    DOTNET_IMAGE: mcr.microsoft.com/dotnet/sdk:6.0

...

build:
    image: $DOTNET_IMAGE

...

tests:
    image: $DOTNET_IMAGE