Use Debian for Buck build image

The new Debian image should be used for all builds
including Buck.

The old Ubuntu 15.x is now obsolete and can be safely
discontinued. Will do that in the follow-up change when
no more images will point to it.

Change-Id: I5c0401a3db1ce407841fb297bff85072204914d7
2 files changed
tree: 1e5e6a2a31e56324194e36a5fe155ca1b96527d2
  1. jenkins/
  2. jenkins-docker/
  3. worker/
  4. .gitignore