fix: caddy resolves own app via ${STACK_NAME}_app on shared proxy net

The caddy on-demand-TLS ask + reverse_proxy used bare host "app", which on the
shared external proxy net collides with every other stack aliasing its main
service "app" (docker DNS returns a foreign endpoint -> cert ask refused -> 000
on warm-canonical promote). Use the fully-qualified swarm service name
${STACK_NAME}_app (caddy {$APP_HOST} env) so caddy resolves THIS stack only.
Established coop-cloud pattern (cf. matrix-synapse, mailu, mumble). Bump
CADDYFILE_VERSION v1->v2.

compose.yml

@@ -52,6 +52,7 @@ services:
       - internal
     environment:
       - DOMAIN=${DOMAIN}
+      - APP_HOST=${STACK_NAME}_app
     configs:
       - source: caddyfile
         target: /etc/caddy/Caddyfile