How to build, push and pull multiple docker contai

I have a docker-compose file which builds two containers, a node app and a ngnix server. Now I would like to automate the build and run process on the server with the help of Gitlab runners. I am pretty new to CI-related stuff so please excuse my approach:

I would want to create multiple repositories on gitlab.com and have a Dockerfile for each one of these. Do I now have to associate a gitlab-runner instance with each of these projects in order to build the image, push it to a docker repo and let the server pull it from there? And then I would have to somehow push the docker-compose file on the server and compose everything from there.

So my questions are:

Am I able to run multiple (2 or 3) gitlab-runner for all of my repos on one server?
Do I need a specific or shared runner and what exactly is the difference?
Why are all tutorials using self hosted Gitlab instances instead of just using gitlab repos (Is it not possible to use gitlab-runner with gitlab.com repos?)
Is it possible to use docker-compose in a gitlab-runner pipeline and just build everything at once?

First of all, you can obviously use GitLab CI/CD features on https://gitlab.com as well as on self hosted GitLab instances. It doesn't change anything, except the host on which you will register your runner:

https://gitlab.com/ in case you uses GitLab without hosting it
https://your-custom-domain/ in case you host your own instance of GitLab

You can add as many runners as you want (I think so, and at least I have 5-6 runners per project without problem). You just need to register each of those runners for your project. See Registering Runners for that.

As for shared runners versus specific runner, I think you should stick to share runners if you wish to try GitLab CI/CD.

Shared Runners on GitLab.com run in autoscale mode and are powered by DigitalOcean. Autoscaling means reduced wait times to spin up builds, and isolated VMs for each project, thus maximizing security.

They're free to use for public open source projects and limited to 2000 CI minutes per month per group for private projects. Read about all GitLab.com plans.

You can install your own runners on literraly any machine though, for example your laptotp. You can deploy it with Docker for a quick start.

Finally, yes you can use docker-compose in a gitlab-ci.yml file if you use ssh executor and have docker-compose install on your server. But I recommend using the docker executor and use docker:dind (Docker in Docker) image

What is Docker in Docker?

Although running Docker inside Docker is generally not recommended, there are > some legitimate use cases, such as development of Docker itself.

Here is an example usage, without docker-compose though:

image: docker:latest

services:
  - name: docker:dind
    command: ["--experimental"]


before_script:
  - apk add --no-cache py-pip      # <-- add python package install pip
  - pip install docker-compose     # <--- add docker-compose 
  - echo "$CI_REGISTRY_PASSWORD" | docker login -u "$CI_REGISTRY_USER" --password-stdin    # <---- Login to your registry

build-master:
  stage: build
  script:
    - docker build --squash --pull -t "$CI_REGISTRY_USER"/"$CI_REGISTRY_IMAGE":latest .
    - docker push "$CI_REGISTRY_USER"/"$CI_REGISTRY_IMAGE":latest
  only:
    - master

build-dev:
  stage: build
  script:
    - docker build --squash --pull -t "$CI_REGISTRY_USER"/"$CI_REGISTRY_IMAGE":"$CI_COMMIT_REF_SLUG" .
    - docker push "$CI_REGISTRY_USER"/"$CI_REGISTRY_IMAGE":"$CI_COMMIT_REF_SLUG"
  except:
    - master

As you can see, I build the Docker image, tag it, then push it to my Docker registry, but you could push to any registry. And of course you could use docker-compose at any time in a script declaration

My Git repository looks like :

/my_repo
|---- .gitignore    
|---- .gitlab-ci.yml    
|---- Dockerfile    
|---- README.md

And the config.toml of my runner looks like:

[[runners]]
  name = "4Gb digital ocean vps"
  url = "https://gitlab.com"
  token = "efnrong44d77a5d40f74fc2ba84d8"
  executor = "docker"
  [runners.docker]
    tls_verify = false
    image = "docker:dind"
    privileged = false
    disable_cache = false
    volumes = ["/var/run/docker.sock:/var/run/docker.sock", "/cache"]
    shm_size = 0
  [runners.cache]

You can take a look at https://docs.gitlab.com/runner/configuration/advanced-configuration.html for more information about Runner configuration.