The Dockerfiles used to build the Nginx images use the environment variable NGINX_VERSION to internally specify the NGINX version to download or build. Is this an implementation detail, subject to change arbitrarily without notice, or can this be considered part of the stable interface the container provides (that can be reliably used to e.g. query the NGINX version in an a custom /docker-entrypoint.d script)? If this is part of the stable interface, I believe this should be documented somewhere.