Make Dockerfile.single a standalone Dockerfile

https://gitlab.com/gitlab-org/gitlab-docs/-/blob/master/dockerfiles/Dockerfile.single (EDIT by @tnir) https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/dockerfiles/single.Dockerfile depends on 3 Dockerfiles, something that complicates things. The order goes like this:

base -> bootstrap -> builder.onbuild -> single

We should make it a standalone Dockerfile.