Unable to pass credential hash for builds

[jeffb-stell]

The existing credential touchpoints in docker-api (Docker.authenticate! and Docker.creds) do not support config file credential hashes being passed in.

Docker::Util.build_config_header attempts to create a useful hash, but an interface that allowed using a closer representation of the map expected by the API would be useful for, as an example, Docker builds that access multiple private repositories.