docker: fix failure to restart app with Docker Compose.

Fix docker-compose sometimes failing to restart the app. Puma's pidfile
was stored in /tmp, which was persisted across restarts, which caused
restarts to fail because the pidfile already existed. The fix is to
mount /tmp as a tmpfs.

docker-compose.yaml

@@ -32,6 +32,8 @@ services:
       - DANBOORU_CANONICAL_URL=http://localhost
     volumes:
       - "danbooru-images:/danbooru/public/data"
+    tmpfs:
+      - /tmp
     depends_on:
       - postgres
     command: ["bash", "-c", "bin/rails db:setup; bin/rails db:migrate && bin/rails server -b 0.0.0.0"]