Briefly,
--tmpfs[=[CONTAINER-DIR[:<OPTIONS>]]]
Mount a temporary filesystem (tmpfs) mount within a container. For example, `--tmpfs /tmp:rw,size=787448k,mode=1777` mounts a tmpfs at /tmp within the container. The mount copies the underlying content into the /tmpfs). The supported options are the same as the Linux default mount flags `rw,noexec,nosuid,nodev,size=65536k`.
More details here:
https://github.com/docker/docker/pull/13587
Former user commented on 2018-06-06T02:52:03.930-0400:
With the reduced investment in docker, we'll not spend time on this improvement. We'll file a new ticket or reopen this if the need comes up again.