Failure if tmp directory does not exist

[benjaminwood]

If for some reason a tmp directory does not already exist with appropriate permissions, docker-compose up will fail to bring the rails container up with the following errors:

rails_1     | touch: cannot touch '/app/tmp/setup.lock': Permission denied
app_name_rails_1 exited with code 1

I believe the tmp directory should exist. But perhaps there is something we can do in the event that it does not.