feat: add support for GitLab CI again

[SimonJasansky]

We want to support GitLab CI in Substrate again. This PR implements the template for the .gitlab-ci.yml file.

Implementation Notes

• GitLab runners (hosted by GitLab) do not allow to run in privileged mode. Thus, it's not possible to use devcontainer up directly, as we do in CI on GitHub.
  • There is a stage build-dev-container that builds and pushes the image to GitLab's container registry, and the stage test then uses that image.
• Dependencies are not installed using the postStartCommand from the .devcontainer configuration directly, but by using uv sync within the started devcontainer in the test stage. It's not ideal to have this code duplicated, but I could not find a good way to directly use the devcontainer command.

Open TODOs

Have stages for:

• lint & test
• build
• publish