Add flags to run "builtin" transport registry

Adding root level flags:

• --builtin-registry (-b): (bool) Use a builtin registry for the transport registry
• --registry-data (-d) Data directory to store registry data

The idea is that this assumes we're running a container with registry on our path that synker can os.Exec and run in the background.

This allows synker to run at the docker build stage instead of the docker run stage easily.