Create "concrete" flavoured helper-image

• The base image contains just git and git-lfs in a way that can be moved to and work from any environment.

  git-lfs is statically compiled.

  Whilst git could be statically compiled (against musl), to enable it to work in any environment, this would prove problematic in FIPS based environments, where we want to load the system's openssl, which will have a certification against the system that it is installed against.

  For this reason, git will be bundled with the glibc it was compiled against, and if a system libssl is available, it will use that. We'll build a wrapper that will negotiate whether to use the bundled openssl lib, the system library, and also which version of openssl (1.1.* or 3.*).

• Add gitlab-runner-helper to this base image.

When concrete mode is therefore enabled, we'll bootstrap the runner helper binary as well as the git dependencies. This means when using concrete, the user provided helper image flavor will no longer be relevant (alpine or ubuntu).