Private docker registry support

[timothyb89]

Support for plain Docker registries should be added. Right now dockerhub repositories determine current image versions using APIs only available from hub.docker.com.

At a minimum we need to support private docker registries (running https://hub.docker.com/_/registry/). This is (very) partially implemented now but needs to be finished. Current version detection will be somewhat worse.

There's also Docker Trusted Registry. It exposes a few more API functions and may be able to return timestamped tags. If the APIs match those on hub.docker.com we can add it to that plugin, otherwise it will need its own.