Add tmpfs to docker-compose

Based on the findings from https://github.com/plankanban/planka/issues/852 a tmpfs location is needed for /app/.tmp to send attachments _if_ you override the user UID/GID.

All of my volume mounts point to a local directory for a system user so I have to override the uid/gid for all of my containers. I believe many others fit into this same scenario, so having this optional text in the docker compose file would be very helpful.

docker-compose.yml

@@ -7,6 +7,10 @@ services:
       - user-avatars:/app/public/user-avatars
       - background-images:/app/public/background-images
       - attachments:/app/private/attachments
+    # Optionally override this to your user/group
+    #user: "1000:1000"
+    #tmpfs:
+    #  - /app/.tmp:mode=770,uid=1000,gid=1000
     ports:
       - 3000:1337
     environment: