botor | Reticulate wrapper on 'boto3 ' with convenient helper | AWS library

I have dotnet solution which consists of console project, webapi project and mysql db. I put them in separate docker images, wrote docker-compose to start and tested on my machine. Next, I wrote a test using FluentDocker, which allows me to start docker-compose programmatically and verify that containers are up and running.

Now I want to do this on Gitlab CI. Previously I used image: mcr.microsoft.com/dotnet/core/sdk:3.1 and run test stage against test project. It worked fine because there was no docker integration. I cant run FluentDocker test on Gitlab CI because the image does not contain docker. So I started researching.

1. Solution to incorporate db in CI job is here https://docs.gitlab.com/ee/ci/docker/using_docker_images.html#what-is-a-service , but I doubt that I can use docker as service.

2. Next is using docker intergration for gitlab runner https://docs.gitlab.com/ee/ci/docker/using_docker_build.html#use-docker-in-docker-workflow-with-docker-executor or https://tomgregory.com/running-docker-in-docker-on-windows/ I cant use that because I use free runner from Gitlab itself and cant configure it. I tried to run docker info command, it fails in my script.

3. I thought of building my own image based on dotnet sdk with docker included, but it seems like a bad idea. And I did not make it working in the end.

4. Solution that seems working is to use dind and start docker-compose: How to run docker-compose inside docker in docker which runs inside gitlab-runner container? or https://gitlab.com/gitlab-org/gitlab-foss/-/issues/30426 But to use it I will need to install dotnet-sdk to build and test my app in before-script section, and sdk is not a small package to download each time.

5. I can try to build my image based on docker:dind and include dotnet sdk there, publish it on dockerhub then use it in gitlab runner. Now it seems to me as the last option.

So, whats the correct approach here?

-----------edit--------------

I made this working! See the very thorough answer from Konrad Botor with dockerfile and yml file. I built my own image with sdk and docker and used it for test stage with dind service linked. My image is hosted on dockerhub, so gitlab downloads it for usage.

Also some notes:

1 - how to use dind as service https://gitlab.com/gitlab-org/gitlab-runner/-/issues/25344

2 - where to get modprobe.sh and docker-entrypoint.sh https://github.com/docker-library/docker (go inside latest release). Very important is to clone repo and copy files from there, because I tried to copy-paste contents and it did not work.

3 - docker-compose repo https://github.com/tmaier/docker-compose/blob/master/Dockerfile

4 - dind example https://gitlab.com/gitlab-examples/docker/-/blob/master/.gitlab-ci.yml