Support for reading Mariadb and Admin password from file when using docker secrets

With this PR, password can be read from docker secrets in both compose as well as swarm environment.

```YAML

secrets:
  mariadb-root-password:
    file: mariadb-root-password.txt
  erpnext-admin-password:
    file: erpnext-admin-password.txt

services:
  erpnext:
    image: frappe/erpnext-worker:${ERPNEXT_VERSION:-v12.5.2}
    environment:
      - SITE_NAME=example.com
      - DB_ROOT_USER=root
      - MARIADB_HOST=mariadb
      - INSTALL_APPS=erpnext
      - FORCE=1
      - REDIS_CACHE=redis-cache:6379
      - REDIS_QUEUE=redis-queue:6379
      - REDIS_SOCKETIO=redis-socketio:6379
      - SOCKETIO_PORT=9000
      - AUTO_MIGRATE=1
      - ADMIN_PASSWORD_FILE=/run/secrets/erpnext-admin-password
      - MYSQL_ROOT_PASSWORD_FILE=/run/secrets/mariadb-root-password
    secrets:
      - erpnext-admin-password
      - mariadb-root-password
    restart: on-failure
    volumes:
      - erpnext-data:/home/frappe/frappe-bench/sites
      - assets-data:/home/frappe/frappe-bench/sites/assets
    links:
      - redis-cache
      - redis-queue
      - redis-socketio
      - mariadb
    depends_on:
      - mariadb
      - redis-cache
      - redis-queue
      - redis-socketio
    networks:
      - erpnext-net
```

Reference: [Addind docker secrets in to your images](https://docs.docker.com/engine/swarm/secrets/#build-support-for-docker-secrets-into-your-images)

Changes to be committed:
	modified:   ../../README.md
	modified:   ../common/commands/new.py
	modified:   ../common/commands/restore_backup.py
	modified:   ../erpnext-nginx/docker-entrypoint.sh

build/common/commands/new.py

@@ -3,10 +3,32 @@ import os, frappe, json
 from frappe.commands.site import _new_site
 from check_connection import get_config, get_site_config
 
+def get_password(env_var, default=None):
+    return os.environ.get(env_var) or _get_password_from_secret(f"{env_var}_FILE") or default
+
+
+def _get_password_from_secret(env_var):
+    """Fetches the secret value from the docker secret file 
+    usually located inside /run/secrets/
+    Arguments:
+        env_var {str} -- Name of the environment variable 
+        containing the path to the secret file.
+    Returns:
+        [str] -- Secret value
+    """
+    passwd = None
+    secret_file_path = os.environ.get(env_var)
+    if secret_file_path:
+        with open(secret_file_path) as secret_file:
+            passwd = secret_file.read().strip()
+
+    return passwd
+
+
 def main():
     site_name = os.environ.get("SITE_NAME", 'site1.localhost')
     mariadb_root_username = os.environ.get("DB_ROOT_USER", 'root')
-    mariadb_root_password = os.environ.get("MYSQL_ROOT_PASSWORD", 'admin')
+    mariadb_root_password = get_password("MYSQL_ROOT_PASSWORD", 'admin')
     force = True if os.environ.get("FORCE", None) else False
     install_apps = os.environ.get("INSTALL_APPS", None)
     install_apps = install_apps.split(',') if install_apps else []
@@ -17,7 +39,7 @@ def main():
         site_name,
         mariadb_root_username=mariadb_root_username,
         mariadb_root_password=mariadb_root_password,
-        admin_password=os.environ.get("ADMIN_PASSWORD", 'admin'),
+        admin_password=get_password("ADMIN_PASSWORD", 'admin'),
         verbose=True,
         install_apps=install_apps,
         source_sql=None,