feat(restic): Add restic backup maintenance stack

Sets up regular backup maintenance for a restic (S3) backend, and
enables global variables for other roles to use for their individual
backup. Example found in nextcloud role.

roles/restic/tasks/main.yml

@@ -0,0 +1,11 @@
+---
+- name: Deploy restic to swarm
+  community.general.docker_stack:
+    name: "{{ stack_name }}"
+    state: present
+    prune: yes
+    compose:
+      - "{{ stack_compose }}"
+  become: true
+  tags:
+    - docker-swarm