add docker pull to makefile
add docker pull to makefile

Bug: skia:
Change-Id: I7088ec7f1b17d6b39ff310141da95bc254932558
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/206647
Reviewed-by: Kevin Lubick <7cdab2cfab351f23814786ba39716e90eed69047@google.com>
Commit-Queue: Nathaniel Nifong <1ce561679465ea4956c6210637580cd7ba456816@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