Use tmpfs for /tmp

This is relevant when docker is used as container runtime as it does not
set /tmp as tmpfs. With podman /tmp is mounted as tmpfs due to
`--read-only-tmpfs` (enabled by default).

Fixes: #3463

docs/setup/install/container.md

@@ -34,6 +34,7 @@ Registry](https://github.com/juanfont/headscale/pkgs/container/headscale). The c
       --name headscale \
       --detach \
       --read-only \
+      --tmpfs /tmp \
       --tmpfs /var/run/headscale \
       --volume "$(pwd)/config:/etc/headscale:ro" \
       --volume "$(pwd)/lib:/var/lib/headscale" \