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.
