docker.tag.long usage

[jmcc0nn3ll]

Currently the jetty9 module tags with a long timestamp format and this pollutes a local docker repository.

We use it for test behavior though so we still need it, but it would be good to perhaps clean this up after running tests.

Is there a reason to have tag.short and tag.long outside of the test usage and does smoking the tag.long at the end of the build raise any red flags?