PAPERLESS_REDIS may be set via docker secrets

Best practice is for redis to be at least password protected: https://redis.io/docs/getting-started/.

Paperless uses `Redis.from_url` (5fe435048b/docker/wait-for-redis.py (L24)) to establish a connection to redis which already enables us to use username/password, e.g. `redis://username:password@redis:6379`.

The redis connection string therefore is a secret and needs to be able to leverage docker secrets, hence this PR.

docker/docker-entrypoint.sh

@@ -62,7 +62,8 @@ initialize() {
 		PAPERLESS_AUTO_LOGIN_USERNAME \
 		PAPERLESS_ADMIN_USER \
 		PAPERLESS_ADMIN_MAIL \
-		PAPERLESS_ADMIN_PASSWORD; do
+		PAPERLESS_ADMIN_PASSWORD \
+		PAPERLESS_REDIS; do
 		# Check for a version of this var with _FILE appended
 		# and convert the contents to the env var value
 		file_env ${env_var}