documented usage of custom build configurations for Docker setup

docker/README.md

@@ -32,7 +32,14 @@ chmod +x docker/local-setup.sh
 docker/local-setup.sh
 ```
 
-Editing the `./.env` file will modify the corresponding variables used in the docker-compose.yml file but **NOT** the environment variables in the container. Editing the `./docker/env` file will change environment variables only in the running container.
+Editing the `./.env` file will modify the corresponding variables used in the docker-compose.yml file but **NOT** the environment variables in the container. 
+Editing the `./docker/env` file will change environment variables only in the running container.
+
+### Custom build config
+
+Our Docker setup supports custom build configurations for the uwsgi contianer via the `CLIENT_DOKERFILE` environment variable (see [compose-env](/docker/compose-env)). The default is `docker/Dockerfile`, which points to a preconfigured [production-like setup](/docker/Dockerfile). For developers who need more control over their setup, we also provide a [configuration](/docker/Dockerfile.dev) that is tailored for local development.
+
+To use a custom build config, change the `CLIENT_DOCKERFILE` variable in the .env file that is automatically created by [local-setup.sh](/docker/local-setup.sh) in the project root.
 
 ## 2. Database File
 Ensure `config/database.yml` has the username and password as defined in [docker/env](docker/env) file. The `config/database.yml` should already be gitignored.