ENV options set to 0 have opposite effect

- `ARANGO_RANDOM_ROOT_PASSWORD=0` enables random root pass.
- `ARANGO_NO_AUTH=0` disables auth

Issues with that:
- unexpected, dangerous behavior
- no way to set some defaults in `.env` to be be overriden in `.env.local`, because `=0` has the effect of `=1`
- docker hub doc does not tell what the defaults are