[docker] Move to testing-slim as the base image.

Also clean out more unneeded packages.

Bug: skia:
Change-Id: Iae642c8878e6d65cda0862e92a881b54634f8f30
Reviewed-on: https://skia-review.googlesource.com/128409
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Joe Gregorio <jcgregorio@google.com>
1 file changed