Add basic caddy swarm role

Added caddy as swarm container. Uses templated Caddyfile which can be
set up for testing purposes for now.
Depends on docker and docker-swarm, and will keep its stuff where
docker-swarm sets itself up.

roles/caddy/templates/docker-stack.yml.j2

@@ -0,0 +1,22 @@
+version: "3.7"
+
+services:
+  app:
+    image: caddy:{{ caddy_version }}
+    ports:
+      - "80:80"
+      - "443:443"
+    volumes:
+      - "{{ caddy_caddyfile_dir }}:/etc/caddy"
+      - data:/data
+      - config:/config
+    networks:
+      - "{{ docker_swarm_public_network_name }}"
+
+volumes:
+  data:
+  config:
+
+networks:
+  "{{ docker_swarm_public_network_name }}":
+    external: true