Fix support for registry auth with Dockerfile build.

Finishes the work from #3099. The original implementation attempted to pass the entire ConfigFile block and then parsed it as an AuthConfig. I believe this isn't feasible because we haven't parsed the Dockerfile in the CLI in order to determine which AuthConfig block is appropriate.

There were two choices, parse the Dockerfile in the CLI and select the proper AuthConfig, or send and parse the entire ConfigFile block and select the proper credentials on the daemon. Since the daemon necessarily has to parse the Dockerfile anyway, I chose the latter.