This adds a HASH file next to the VERSION file in the image
context folders for prebuilt docker images. And uses the
HASH for referencing the image in the tasks created by
the decision task.
This way docker will validate the image hash when pulling it
in production. Thus, attackers won't be able to inject code
by compromising the remote docker registries we use to store
prebuilt images. Further more, this makes validation of the
Chain-Of-Trust artifacts easier as this eliminates the need
for whitelists and hash validation.
MozReview-Commit-ID: FD3B9MyeU9Q
e09600726d