Dedicated Docker Stage

Create a dedicated Docker stage for the docker CI file.

• Create a new docker stage
• Remove the existing docker-* stages
• All Docker jobs should be part of the docker stage
• Make the Docker jobs dependent on each other via the needs flag
• Update all projects which are overwriting the stages

Apart from that, also add simple testing at the end by following these guidelines:

• Test the TEST_URLS (space separated) via curl or wget for a HTTP 200
• If no explicit TEST_URLS is set, use the environment URL as default
• Document the TEST_URLS variable (incl. default fallback)
• If something fails, it should be clear which URL failed

Note:

• It's probably enough to test only the headers (e.g. HEAD instead of GET)