Add a helper function for label schema fields useful for reproducibility

[nuest]

Add a helper function that automatically adds labels that are useful for users of an image to know in the context of reproducibility, see http://label-schema.org/rc1/ for full spec.

• [ ] docker.cmd
• [ ] docker.cmd.help
• [ ] docker.params (if any... see #64)

Consider making these labels even easier via an argument in dockerfile().