As bug 1291940 shows, aufs can be a real performance drag. Most performance critical paths in containers are caches. This allows the use of the host filesystem (instead of aufs), which is much faster under load. Unfortunately, most caches are stripped on Try. This means paths that would be served from the fast host filesystem are now being handled by aufs. This can make Try jobs slower than non-Try jobs. This commit declares Docker volumes for popular cache paths. If the cache is present, the cache takes precedence over the declared volume. If the cache isn't present, Docker falls back to an empty volume that's used for the task and only the task. It's worth noting that desktop-build is updated redundantly with centos6-build. We currently can't rebuild centos6-build because of a determinism issue (bug 1272629) breaking valgrind. So, I work around this by declaring the volume in the top-most "desktop-build" layer. This hack can be removed once we merge the images. Review commit: https://reviewboard.mozilla.org/r/70112/diff/#index_header See other reviews: https://reviewboard.mozilla.org/r/70112/