Added "AUTO_RECEIVE_SCHEDULE" parameter to docker-compose.yml file

* signal-cli recommends to call "receive" on a regular basis. In case
  "receive" is not already called periodically by the user application,
  add the option to invoke it at a specific schedule from within the
  docker container.

  The "AUTO_RECEIVE_SCHEDULE" parameter takes a cron schedule expression
  and calls "receive" at the given time.

see #129

docker-compose.yml

@@ -2,9 +2,9 @@ version: "3"
 services:
   signal-cli-rest-api:
     image: bbernhard/signal-cli-rest-api:latest
-    #build: "."
     environment:
       - USE_NATIVE=0
+      #- AUTO_RECEIVE_SCHEDULE=0 22 * * *
       - PORT=8080
     ports:
       - "8080:8080" #map docker port 8080 to host port 8080.