Dockerfile: switch to "skia" user at end
Dockerfile: switch to "skia" user at end

Change-Id: Ie5558e0fd84fcba0aec9c5f6cfd83eeea089bf31
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/212220
Commit-Queue: Eric Boren <0e499112533c8544f0505ea0d08394fb5ad7d8fa@google.com>
Auto-Submit: Eric Boren <0e499112533c8544f0505ea0d08394fb5ad7d8fa@google.com>
Reviewed-by: Kevin Lubick <7cdab2cfab351f23814786ba39716e90eed69047@google.com>

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot