Fix mastodon user not being owner of tmp folder in Dockerfile (#28137)

Dockerfile

@@ -247,7 +247,9 @@ RUN \
 RUN \
 # Pre-create and chown system volume to Mastodon user
   mkdir -p /opt/mastodon/public/system; \
-  chown mastodon:mastodon /opt/mastodon/public/system;
+  chown mastodon:mastodon /opt/mastodon/public/system; \
+# Set Mastodon user as owner of tmp folder
+  chown -R mastodon:mastodon /opt/mastodon/tmp;
 
 # Set the running user for resulting container
 USER mastodon