Support expanded user configuration via `REGISTRY_USERS` and `REGISTRY_RO_USERS`.

`REGISTRY_USERNAME`/`REGISTRY_PASSWORD` are good for getting started but limiting push access to just where it's needed is preferable. This PR introduces `REGISTRY_USERS` and `REGISTRY_RO_USERS` which allow listing of `username:password` pairs. `RO` users have read-only access with the intention they can be used by automation for pulling images. The credentials in `REGISTRY_USERNAME`/`REGISTRY_PASSWORD` and listed in `REGISTRY_USERS` have read and write access.

Longer term it would be nice to be able to wrap access to the registry in something that authenticates and authorizes access more dynamically, such as against postgres or DynamoDB.