docker: Use a volume for backup configuration

Otherwise we need to rebuild the image. Thanks @duez.
See !877

docker/backup/Dockerfile

@@ -4,9 +4,9 @@ RUN apt-get update && apt-get install -y mysql-client zip
 
 RUN touch /var/log/cron.log
 
-ADD ./conf/backup-cron /etc/cron.d/backup-cron
+RUN ln -sf /etc/backup/backup-cron /etc/cron.d/backup-cron
 
-ADD ./conf/backup.cnf /root/.my.cnf
+RUN ln -sf /etc/backup/backup.cnf /root/.my.cnf
 
 RUN ln -sf /dev/stdout /var/log/cron.log && ln -sf /dev/stderr /var/log/cron.log
 

docker/docker-compose.yml

@@ -73,6 +73,8 @@ services:
         build: ./backup
         volumes_from:
             - uwsgi
+        volumes:
+            - ./backup/conf:/etc/backup
         depends_on:
             - mysql
         links: