Added WORDPRESS_DB_PASSWORD_FILE to readme

resolves issue #225

wordpress/content.md

@@ -17,12 +17,19 @@ The following environment variables are also honored for configuring your WordPr
 -	`-e WORDPRESS_DB_HOST=...` (defaults to the IP and port of the linked `mysql` container)
 -	`-e WORDPRESS_DB_USER=...` (defaults to "root")
 -	`-e WORDPRESS_DB_PASSWORD=...` (defaults to the value of the `MYSQL_ROOT_PASSWORD` environment variable from the linked `mysql` container)
+-	`-e WORDPRESS_DB_PASSWORD_FILE=...` (for use with Docker secrets, see below)
 -	`-e WORDPRESS_DB_NAME=...` (defaults to "wordpress")
 -	`-e WORDPRESS_TABLE_PREFIX=...` (defaults to "", only set this when you need to override the default table prefix in wp-config.php)
 -	`-e WORDPRESS_AUTH_KEY=...`, `-e WORDPRESS_SECURE_AUTH_KEY=...`, `-e WORDPRESS_LOGGED_IN_KEY=...`, `-e WORDPRESS_NONCE_KEY=...`, `-e WORDPRESS_AUTH_SALT=...`, `-e WORDPRESS_SECURE_AUTH_SALT=...`, `-e WORDPRESS_LOGGED_IN_SALT=...`, `-e WORDPRESS_NONCE_SALT=...` (default to unique random SHA1s)
 
 If the `WORDPRESS_DB_NAME` specified does not already exist on the given MySQL server, it will be created automatically upon startup of the `%%REPO%%` container, provided that the `WORDPRESS_DB_USER` specified has the necessary permissions to create it.
 
+As an alternative to passing the database password via environment variables, `WORDPRESS_DB_PASSWORD_FILE` may be used instead of `WORDPRESS_DB_PASSWORD`, causing the initialization script to load the password from a file present in the container. In particular, this can be used to load passwords from Docker secrets stored in `/run/secrets/<secret_name>` files. For example:
+
+```console
+$ docker run --name some-%%REPO%% -e WORDPRESS_DB_PASSWORD_FILE=/run/secrets/db_password -d %%REPO%%
+```
+
 If you'd like to be able to access the instance from the host without the container's IP, standard port mappings can be used:
 
 ```console