Allow specifying the location of the `config/danbooru_local_config.rb`
file with the DANBOORU_CONFIG_FILE environment variable. For example:
DANBOORU_CONFIG_FILE=/etc/danbooru/danbooru_local_config.rb bin/rails server
This is useful in Kubernetes because it lets us mount a directory
containing the config file without it clobbering everything else in the
config/ directory.