Step 3 of the 12 Factor app is to store config in the environment. For Keystone, the set of configuration options is controlled by the keystone.conf file. In an earlier attempt at containerizing the scripts used to configure Keystone, I had passed an environment variable in to the script that would then be written to the configuration file. I realize now that I want the whole keystone.conf external to the application. This allow me to set any of the configuration options without changing the code in the container. More importantly, it allows me to make the configuration information immutable inside the container, so that the applications cannot be hacked to change their own configuration options.