docker.py/build: support -t and -f arguments

The docker.py command line is subtly different from docker and podman's,
in that the tag and Dockerfile are passed via positional arguments.
Remove this gratuitous difference and just parse -f and -t.

-f was previously used by --extra-files, only keep the long option.

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>