Update cloud/docker: if the image name containes a repository, strip it.

When using repositories other than the main one at docker.io, the image name contains the repo name (which itself contains ":" as a separator between domain and port). We don't really care about it here, so just get rid of it before looking at the image name.
Merge pull request #4927 from intellectronica/intellectronica/docker-identify-private-repos-backport

Update cloud/docker: if the image name containes a repository, strip it.