Support for ADD <git ref> in Containerfile via SSH address

[ctlong]

This is an extension of #4974. As of the completion of that issue, the following is supported:

ADD https://github.com/user/repo.git /mydir/

However, buildah's matching only works on HTTP addresses, while Docker allows SSH addresses to be used as well: https://docs.docker.com/reference/dockerfile/#adding-files-from-a-git-repository. This is particularly useful when dealing with private repos, which are typically cloned using SSH key authentication.

I propose adding support for the git@ prefix in SRC to signal that SRC is a git repo to be cloned using SSH.

[danishprakash]

/assign