Add Linux os_islocaldir_alt for Docker.
(right now, specifically only for Docker, but could potentially be used for any crazy container impl where the local filesystem doesn't appear local to df -l. From the commit:) Some environments do not give us a valid / mount (like Docker); thus, df -l does not work. Thus we must rely on /proc/mounts to tell us if a dir is local or not. Well, ok, /proc/mounts is hard (and potential bind mount chains would make it harder). So instead, we assume that / is local, (well, we check to ensure it is not NFS in /proc/mounts), and use os_samefs above to ensure that $dir is on the same device as /. If that is true, that is good enough to call it a local dir.
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