Feat: Adds template for Gitlab CI

[vibalre]

Changes

Related #350 Closes Appsilon Internal Task

• Adds dot.gitlab-ci.yml template
• Adds dot.gitlab-ci.Dockerfile template

How to test

• I cloned and orphaned rhino-showcase in this GitLab repository to run tests.
• rhino-showcase-gitlab pipeline shows all the failed and passed pipelines.

[jakubnowicki]

Since the complexity of this is higher than in the case of GitHub Actions (mainly, the Docker setup), my recommendation is to move files to the examples directory and provide a documentation entry (a how-to guide) on how to set the CI for GitLab. @kamilzyla WDYT?