Fix Docker setup

This commit fixes the Docker setup and also the CI build.

Also the contributing guide is updated with the new information related
to Docker. The CONTRIBUTING.md file is used to comply with GitHub
guidelines to help other developers to contribute.

docker-compose.yml

@@ -3,17 +3,18 @@ services:
   db:
     image: mysql:5.7
     environment:
-      MYSQL_ALLOW_EMPTY_PASSWORD: '1'
+      MYSQL_ALLOW_EMPTY_PASSWORD: 1
+      MYSQL_DATABASE: ${TRACKS_DB:-tracks}
     volumes:
-      - "db-data:/var/lib/mysql"
+      - db-data:/var/lib/mysql
   web:
     build: .
+    command: rails s
     volumes:
-      - "${VOLUME:-.:/app/workdir}"
+      - ${VOLUME:-.:/app}
     ports:
-      - "3000:3000"
+      - 3000:3000
     depends_on:
       - db
 volumes:
   db-data:
-