Minimal Dockerfile could potentially use headless JDK (or even JRE)

[sjbaker47]

In the base image ubuntu/minimal/Dockerfile, the much larger package openjdk-8-jdk is used instead of openjdk-8-jdk-headless. Since the typical use cases of Databricks instances are interactive notebooks and unattended jobs, and many image processing features are in the 'headless' package anyway, I'd be surprised if anyone requires the dependencies in the full version. The space savings are significant: just the X11 DRI drivers that get pulled in are themselves ~350MB.

Also, I've done some preliminary testing, and it even seems like Scala works with the JRE (not JDK) version of this package. I'm not sure if there's some edge case that requires the javac compiler to be around, though.