This includes adding TASKCLUSTER_VOLUMES to docker image builds directly. The
env variable is not added as part of the task transform because `run-task` is
not in payload.command. In fact, build-image.sh calls run-task after doing
some other housekeeping.
Ideally image builds would be turned into jobs and all of this would occur
automatically, but that turns out to be quite a bit too complex for this
incidental fix -- perhaps best solved in another bug.
MozReview-Commit-ID: FYHvafJras7
854c8fee05