Simplify Docker service names for Restic

This simplifies containernames such as "restic_backup_restic-backup_1"
to "restic_backup_1".

roles/docker/tasks/services/restic_backup.yml

@@ -49,12 +49,12 @@
 
 - name: Setup restic backup
   docker_compose:
-    project_name: restic_backup
+    project_name: restic
     pull: true
     definition:
       version: '3.6'
       services:
-        restic-backup:
+        backup:
           image: mazzolino/restic:{{ services.restic.version }}
           restart: always
           environment:
@@ -77,7 +77,7 @@
             - "{{ services.restic.volume_folder }}/ssh:/run/secrets/.ssh:ro"
             - /docker-volumes:/mnt/volumes:ro
 
-        restic-prune:
+        prune:
           image: "mazzolino/restic:{{ services.restic.version }}"
           environment:
             RUN_ON_STARTUP: "false"