The command helpfully downloads a docker image created by a taskcluster task,
which can be helpful when trying to replicate subtle bugs that only occur
in-tree.
This also fixes a bug in hashing Dockerfile directories, where the full
pathname was taken into account. While this pathname is consistent from
decision task to decision task, it is not consistent with developers' home
directories. This change omits the directory prefix, which will cause a
one-time shift in all directory hashes.
MozReview-Commit-ID: EamQzUGG5qY
db6e8b524c