task_driver docker - Fix race.
task_driver docker - Fix race.

Bug: skia:9535
Change-Id: I2dbb750ed0e9f214891699c4a0006921d291de5f
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/248925
Auto-Submit: Joe Gregorio <b9cf2471ff6d33504cafe8e6f356b732b2cadced@google.com>
Reviewed-by: Ben Wagner aka dogben <3ef7217be91069877d94f7907ce5479000772cd3@google.com>
Commit-Queue: Ben Wagner aka dogben <3ef7217be91069877d94f7907ce5479000772cd3@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