Publish distroless docker image

[istreeter]

A docker image based on gcr.io/distroless/java brings in fewer dependencies and should be less work to maintain when addressing potential security vulnerabilities.

Distroless does not even come with a shell, it is slightly more awkward to run -- cannot take advantage of the sbt-native-packager startup script, like providing java options on the command line. For this reason, we should publish this in addition to the main docker image, not instead of.