Have imagebuilder tag multi-tag images pushed to DockerHub

Currently imagebuilder tags images with a single git commit SHA hash, rather than using the branch or VERSION file information. See document for notes on version format:

https://docs.google.com/document/d/1wzNOZHGUAT0Y80qDnANSdh9DQJcJBcK3HVNQbtwdlxo/edit

Come up with a scheme that will tag images of a `master` or `6.0.0-dev` or similar tags that can be used to pull images without having to look up those hashes. Also tag with a composite `master-<githash>` as well, to pull specific versions.

This is in support of the Helm process, to allow the docker "pull newest image version" functionality.