setup stretch-slim base image to allow installing openjdk

[eighthave]

The stretch-slim base image removes the man pages, docs, locales, etc. Some package install processes will fail if the dirs are not present, like /usr/share/man/man1. This includes default-jdk-headless, which someone would install if they wanted to run a gradle build in the Docker image before starting the emulator.

The mkdir hack comes from:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=863199

Here's an example of running a build in this image:
https://gitlab.com/eighthave/ci-images-emulator-kvm/-/jobs/1052060212

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[pokowaka]

Would you mind signing the cla? That way we can merge it in

[eighthave]

@googlebot I signed it!

[eighthave]

I see you just refactored the Dockerfile template handling. I think it probably makes sense to just include these changes in the Dockerfile templates. I can't figure out which Docker base image it being used now though.