Readme: Improve docker-compose sample and describe cron job schedule (#7)

* Readme: Improve docker-compose sample

- Add attributes "version" and "services" to make it a docker-compose.yml
- Add mariadb service with preconfigured user and database

Resolves https://github.com/gutmensch/docker-dmarc-report/issues/1

* Readme: Describe cron job schedule

It took me a while to figure out why nothing happened after starting the
container. This should help new users to understand that they only have
to wait for the cron job to start.

README.md

@@ -18,33 +18,52 @@ docker pull gutmensch/dmarc-report
 docker run -e ... -ti gutmensch/dmarc-report
 ```
 
+New dmarc reports will be fetched every 15 minutes past the hour, every hour. Therefore it can take up to one hour for the first report to be fetched.
+
 ## Versions for last build latest
 dmarcts report viewer: 2019-01-04
 
 dmarcts report parser: 2019-01-04
 
 ## Sample docker compose / Environment variables
-The variables should be self-explanatory. Make sure to create the DB and IMAP folders before the cron job runs!
-```
+The variables should be self-explanatory. Make sure to create the IMAP folders before the cron job runs!
+
+**docker-compose.yml**
+```yaml
+version: '3.6'
+
+services:
   dmarc-report:
     image: "gutmensch/dmarc-report:latest"
     hostname: dmarc-report
     container_name: dmarc-report
+    depends_on:
+      - db
+    ports:
+      - "80:80"
     environment:
-    - "REPORT_DB_HOST=mysql"
+      - "REPORT_DB_HOST=db"
       - "REPORT_DB_NAME=dmarc_report"
       - "REPORT_DB_USER=dmarc_report"
-    - "REPORT_DB_PASS=foobar"
+      - "REPORT_DB_PASS=dbpassword"
       - "PARSER_IMAP_SERVER_WITH_PORT=mail:143"
-    - "PARSER_IMAP_USER=foobar@example.com
+      - "PARSER_IMAP_USER=foobar@example.com"
       - "PARSER_IMAP_PASS=foobar"
       - "PARSER_IMAP_READ_FOLDER=Inbox"
       - "PARSER_IMAP_MOVE_FOLDER=processed"
+
+  db:
+    image: mariadb:10
+    environment:
+      - "MYSQL_ROOT_PASSWORD=dbrootpassword"
+      - "MYSQL_DATABASE=dmarc_report"
+      - "MYSQL_USER=dmarc_report"
+      - "MYSQL_PASSWORD=dbpassword"
 ```
 
 ## Optional extended configuration
 Use SSL instead of default TLS. Set both to 0 to turn off encryption. (not recommended)
-```
+```yaml
     - "PARSER_IMAP_SSL=1"
     - "PARSER_IMAP_TLS=0"
 ```