Before, the build root was not in a Docker cache or volume. With
current Docker works, that meant AUFS. We know AUFS is slow under
I/O load and can cause random failures due to missing data after
writes.
This commit changes the build root to a known Docker volume, which
will be backed by EXT4 and won't have the problems of AUFS.
MozReview-Commit-ID: 6WOH0yednAv
--HG--
extra : rebase_source : bbff0f00f55acdbe068fdf617a7903b8a303c397
4783c071f3