Fix SMTP configuration to use Cloudron sendmail hostname

Change SMTP host from localhost/127.0.0.1 to 'mail' as per Cloudron
sendmail addon documentation. The sendmail addon provides a local SMTP
relay accessible via hostname 'mail' on port 2525.

CloudronManifest.json

@@ -7,7 +7,7 @@
   "contactEmail": "contact@ente.io",
   "website": "https://ente.io",
   "tagline": "Open source, end-to-end encrypted photo backup",
-  "version": "0.3.5",
+  "version": "0.3.6",
   "upstreamVersion": "git-main",
   "healthCheckPath": "/health",
   "httpPort": 3080,

start.sh

@@ -122,7 +122,7 @@ HASH_KEY_FILE="$SECRETS_DIR/hash_key"
 JWT_SECRET_FILE="$SECRETS_DIR/jwt_secret"
 SESSION_SECRET_FILE="$SECRETS_DIR/session_secret"
 
-SMTP_HOST="localhost"
+SMTP_HOST="mail"
 SMTP_PORT="2525"
 SMTP_ENCRYPTION=""
 SMTP_USERNAME=""