Thanks Jonathan, that bit on file locking is I think just what I was looking for. We've seen the same warning occur on instances running exclusively on Linux-based hosts. Only seems to happen though if the container is sharing volumes with the host, so probably has something to do with how Docker handles shared volumes.
Will definitely try switching to advisory-based locks and see how that works out. I don't personally run containers on any shared / NFS based systems, but wonder if advisory locks are going to create problems there for any users who do?
I do see these errors every time a session crashes as well. I wonder if shortening the expiration times would work (guess that only applies to link-based locks anyhow).