Build time user and uid/gid

[pdmack]

Picks up from #68
Closes #22

Notes:

• can specify an arbitrary user, uid/gid when building release image
• default is still root (0:0)
• docker run --user ... with an arbitrary uid/gid still allows that user to run the CLI tool
• smoke tested with file-based DFP inf-pytorch pipeline

Unresolved questions:

• should the build uid even be a sudoer?