update advanced-config/readme with secrets

Author: ahgraber

docs/advanced-config/README.md

@@ -1,5 +1,66 @@
 # Advanced Configuration
 
+## Docker Secrets
+
+This image supports the use of Docker secrets to import from file and keep sensitive usernames or passwords from being passed or preserved in plaintext.
+
+You can set any environment variable from a file by appending `__FILE` (double-underscore FILE) to the environmental variable name.
+
+```yml
+version: "3.7"
+
+secrets:
+  # Secrets are single-line text files where the sole content is the secret
+  # Paths in this example assume that secrets are kept in local folder called ".secrets"
+  DB_ROOT_PWD:
+    file: .secrets/db_root_pwd.txt
+  MYSQL_PWD:
+    file: .secrets/mysql_pwd.txt
+
+services:
+  app:
+    image: 'jc21/nginx-proxy-manager:latest'
+    restart: always
+    ports:
+      # Public HTTP Port:
+      - '80:80'
+      # Public HTTPS Port:
+      - '443:443'
+      # Admin Web Port:
+      - '81:81'
+    environment:
+      # These are the settings to access your db
+      DB_MYSQL_HOST: "db"
+      DB_MYSQL_PORT: 3306
+      DB_MYSQL_USER: "npm"
+      # DB_MYSQL_PASSWORD: "npm"  # use secret instead
+      DB_MYSQL_PASSWORD__FILE: /run/secrets/MYSQL_PWD 
+      DB_MYSQL_NAME: "npm"
+      # If you would rather use Sqlite uncomment this
+      # and remove all DB_MYSQL_* lines above
+      # DB_SQLITE_FILE: "/data/database.sqlite"
+      # Uncomment this if IPv6 is not enabled on your host
+      # DISABLE_IPV6: 'true'
+    volumes:
+      - ./data:/data
+      - ./letsencrypt:/etc/letsencrypt
+    depends_on:
+      - db
+  db:
+    image: jc21/mariadb-aria
+    restart: always
+    environment:
+      # MYSQL_ROOT_PASSWORD: "npm"  # use secret instead
+      MYSQL_ROOT_PASSWORD__FILE: /run/secrets/DB_ROOT_PWD
+      MYSQL_DATABASE: "npm"
+      MYSQL_USER: "npm"
+      # MYSQL_PASSWORD: "npm"  # use secret instead
+      MYSQL_PASSWORD__FILE: /run/secrets/MYSQL_PWD 
+    volumes:
+      - ./data/mysql:/var/lib/mysql
+```
+
+
 ## Disabling IPv6
 
 On some docker hosts IPv6 may not be enabled. In these cases, the following message may be seen in the log: