Before this commit, when sharing the home directory with the container, our policy was to also share top-level directories that don't technically interfere with the container runtime, but only if one of these is true: either they're described by the FHS in terms that make sharing them reasonable (like /home, /media, /srv), or they're used by a major OS for a purpose that is consistent with us sharing them (like the non-FHS /run/media, although that isn't top-level).
However, some Steam users install games in arbitrary non-FHS top-level directories like /games, /storage or /stuff. We can't really tell whether people would expect to share these directories or not - for every directory for which sharing would be reasonable (like /games or /storage), there's a directory for which sharing would be unexpected (like /os or /lost+found or /corporate-secrets). We can either share none of these, or all of them. This commit shares all of them.
Based on !485 (merged) - only the last commit should be considered here. We discussed doing this for steamrt/tasks#149, but were uneasy about actually doing it.
I wanted to put this up as a proposal, even if we're going to reject it. The preparatory work in !485 (merged) seems good, so my inclination is to merge !485 (merged) (assuming reviewers are happy with it) but reject !486 (closed).