Provide multiple versions based on different TexLive versions

This makes it possible to reuse the dockerfile to create multiple docker-images based on different TL-Versions in CI. This is needed as some packages change commands with different version of TL.

An example is that fontawesome5 changed the name of the icons slightly between those version, leading to a situation where I can either build the document in CI or locally when using the current version locally. This problem is addressed by offering multiple versions of the docker-image.

Also this PR changes the behaviour of CI to now build the docker-image automatically on every commit that passes the test. By this, the docker-image always reflects the latest changes of the repo and cannot be forgotten to be updated.