Make tmpdir volume available to web containers to work around file buffering on disk
As documented at gitlab-org/gitlab#11548 (comment 615578282)
Certain web requests leave temp/buffer files on disk, which in our default container configuration could fill up an entire Kubernetes node root disk volume, essentially crashing the whole node.
By using a tmpdir volume inside the manifest for the pod itself, we will instead have guaranteed space for this and Kubernetes itself will just kill the pod if the disk space is used, which overall is more desirable.
The long-term solution for the problem will be fixed with gitlab-org&6396 (closed). Once delivered we should review whether to remove our solution.
Edited by Amy Phillips