feat(shepherd): Add auto update shepherd role

Deprecates diun as it provides a simpler implementation for docker
swarm. Mark any containers you want auto updated with
`shepherd.autoupdate=true` and the rest with
`shepherd.autoupdate=false`. Everything untagged will not be watched (by
default), though this can be changed by setting the ansible default
variable `shepherd_filter_services: `.

roles/shepherd/tasks/main.yml

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