Ensure stacks can inject their upstream to caddy

Created bind volume for caddy which takes the json fragments which will
later be injected through the caddy api to dynamically set their
individual reverse proxy addresses.

This is not entirely necessary if the injection should be done as a
variable instead, but makes it easier to change routes and especially
more complicated ones (I'm thinking of gitea with ssh access e.g.).

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

@@ -8,6 +8,7 @@ services:
       - "443:443"
     volumes:
       - "{{ caddy_caddyfile_dir }}:/etc/caddy"
+      - "{{ docker_stack_files_dir }}:/stacks:ro"
       - data:/data
       - config:/config
     networks: