Description of problem:

Systemd in container needs /run to be a separate volume or it complains

# docker run --rm -ti -v /sys/fs/cgroup:/sys/fs/cgroup:ro rhel7 /usr/sbin/init
Failed to mount tmpfs at /run: Operation not permitted
[!!!!!!] Failed to mount API filesystems, freezing.

So in my FreeIPA server Dockerfiles I do

VOLUME /run

to minimize the number of options that users have to pass docker run (avoid having to specify -v /run).

This works but with Docker 1.10, it'd be good to be able to use --tmpfs for that volume.

Version-Release number of selected component (if applicable):

docker-common-1.9.1-38.el7.x86_64
docker-latest-1.10.3-19.el7.x86_64

How reproducible:

Deterministic.

Steps to Reproduce:
1. Have Dockerfile:

FROM rhel7
VOLUME /run

2. Built image with docker build -t test-tmpfs .
3. Run docker run --rm -ti test-tmpfs true, see it pass.
4. Run docker run --rm -ti --tmpfs /run test-tmpfs true.

Actual results:

docker: Error response from daemon: Cannot start container 4086ca4dc1a375346d28a12ab34c45448f898614b21c6960f42008fa76e19639: Duplicate mount point '/run'.

Expected results:

No error, Docker just using tmpfs for the /run volume.

Additional info:

This is primarily compatibility and upgrade issue. Since it does not seem to be possible to specify the TMPFS in Dockerfile, it needs to be specified as docker run parameter. But it's not available with docker 1.9-, so it has to be optional. It be good if people on docker 1.10+ could take the advantage of --tmpfs, and people with older dockers could just keep using the built-in VOLUME /run definition.