Improve Docker cache invalidation

[martinmladenov]

The current way of cache invalidation when cloning the repo in the docker image is very strange, perhaps we should create an issue to improve it. Specifically, ADD supports Git repositories natively, mayhaps we can replace the cache-buster and clone with a singular repository ADD.

Originally posted by @Arraying in https://github.com/SERG-Delft/andy/issues/261#issuecomment-1667561011