fix(app): use stack-prefixed hostnames for redis and db

The app service was missing DISCOURSE_REDIS_HOST entirely, so the
bitnami image defaulted to the bare hostname 'redis', which does not
resolve when the service is attached to multiple overlay networks
(proxy + internal) — causing a restart loop.

sidekiq had the same problem worse: both DISCOURSE_DATABASE_HOST=db
and DISCOURSE_REDIS_HOST=redis used bare hostnames instead of the
stack-prefixed service names. On a swarm stack the reliable DNS name
is ${STACK_NAME}_db / ${STACK_NAME}_redis.

compose.yml

@@ -16,6 +16,8 @@ services:
       - DISCOURSE_DATABASE_USER=discourse
       - DISCOURSE_HOST=${DOMAIN}
       - DISCOURSE_NOTIFICATION_EMAIL
+      # bare "redis" doesn't resolve when app is on multiple networks; use stack-prefixed name
+      - DISCOURSE_REDIS_HOST=${STACK_NAME}_redis
       - DISCOURSE_SMTP_AUTH
       - DISCOURSE_SMTP_HOST
       - DISCOURSE_SMTP_PORT
@@ -100,13 +102,13 @@ services:
       - db_password
     environment:
       - ALLOW_EMPTY_PASSWORD=yes
-      - DISCOURSE_DATABASE_HOST=db
+      - DISCOURSE_DATABASE_HOST=${STACK_NAME}_db
       - DISCOURSE_DATABASE_NAME=discourse
       - DISCOURSE_DATABASE_PASSWORD_FILE=/run/secrets/db_password
       - DISCOURSE_DATABASE_PORT_NUMBER=5432
       - DISCOURSE_DATABASE_USER=discourse
       - DISCOURSE_HOST=${DOMAIN}
-      - DISCOURSE_REDIS_HOST=redis
+      - DISCOURSE_REDIS_HOST=${STACK_NAME}_redis
       - DISCOURSE_REDIS_PORT_NUMBER=6379
       - DISCOURSE_SMTP_HOST
       - DISCOURSE_SMTP_PORT