Unicorn: add shared /tmp
Add a shared /tmp mount to the unicorn and workhorse containers.
This addresses #1647 (closed), and customer issues.
Changes in GitLab 12.4.0 rails / workhorse (gitlab-org/gitlab!18135 (merged)) resulted in the hard requirement of this shared directory.
It is using emptyDir: {} to ensure the use of a host-backed working
directory, as medium: Memory is dangerous due to the consumption
of the disk counting against resources.limits.memory
Closes #1647 (closed)