fix: make Docker service label configurable for staging

The Kamal service label was hardcoded to 'kelas', causing staging
deployment to fail. Now uses SERVICE_NAME build arg with 'kelas' as
default, and staging workflow passes 'kelas-staging'.

Author: zainfathoni

.github/workflows/deploy-staging.yml

@@ -47,6 +47,8 @@ jobs:
           tags: |
             ghcr.io/zainfathoni/kelas.rumahberbagi.com:staging-${{ github.sha }}
             ghcr.io/zainfathoni/kelas.rumahberbagi.com:staging-latest
+          build-args: |
+            SERVICE_NAME=kelas-staging
           cache-from: type=gha
           cache-to: type=gha,mode=max
           platforms: linux/amd64

Dockerfile

@@ -71,8 +71,9 @@ EXPOSE 3000
 ENV NODE_ENV=production
 ENV PORT=3000
 
-# Kamal requires service label
-LABEL service="kelas"
+# Kamal requires service label (configurable for staging)
+ARG SERVICE_NAME=kelas
+LABEL service="${SERVICE_NAME}"
 
 # Start the application
 # Using explicit path for CJS compatibility with Node 24