Docker: use a volume to store translations

[aalemayhu]

The current implementation stores stuff either in /tmp/ or $STORAGE_DIR. Since I am not setting the environment variable anywhere, we are only using a temporary directory. This has the downside that whenever the container is updated all of the translated rules are gone. Yes, the rules can be easily recreated, but broken urls is a bug.

[aalemayhu]

• Update Makefile.
• Figure out how to do it in Dokku.