MariaDB default encoding utf8mb4 and maintenance

* MariaDB encoding is set when creating DB through the entrypoint
* Cleaning Dockerfile
  * Removed `POSTGRES_USER`
  * Combined run commands
  * Changed deprecated `MAINTAINER` to `LABEL`

README.md

@@ -91,7 +91,7 @@ $ docker exec -t NAME_OR_ID_OF_YOUR_WALLABAG_CONTAINER /var/www/wallabag/bin/con
 It's a good way to use [docker-compose](https://docs.docker.com/compose/). Example:
 
 ```
-version: '2'
+version: '3'
 services:
   wallabag:
     image: wallabag/wallabag