[$] Container-aware filesystems

We are getting closer to being able to do unprivileged mounts insidecontainers, but there are still some pieces that do not work well in thatscenario. In particular, the user IDs (and group IDs) that are embeddedinto filesystem images are problematic for this use case. James Bottomleyled a discussion on the problem in a session at the 2017 Linux Storage,Filesystem, and Memory-Management Summit.