Optimize CI, add cache

[Syndesi]

The current CI does not take advantage of caching. This is especially a problem for the "Build Docker Image" step, which takes around 11 minutes.

The following changes should be tested and implemented:

• Enable Docker image cache for non-main branches.
• Build the Docker image automatically once per week.
• Rebuild the Docker image after a change got merged into main which affects the Docker image.
• Use the cached Docker image in non-main CI runs when there are no changes to the Docker image itself.
• Keep building the image from scratch on non-main CI runs when there are changes to the Docker image.
• Investigate which kind of changes affect the Docker image, e.g.:
  • Changes to docker/
  • Changes to composer.json should not require a newly built image automatically, as Composer dependencies are installed inside the test CI automatically. If Composer dependencies require changes to the Docker image, then the Dockerfile itself has to be updated itself, which triggers a change to docker/, which already rebuilds the image.