fix(commerce-backend): agregar dummies de build para vars de Google OAuth

npm run build (medusa build) carga medusa-config.ts, que ahora exige
GOOGLE_CLIENT_ID/GOOGLE_CLIENT_SECRET/GOOGLE_CALLBACK_URL via required().
Sin valores dummy en el builder stage, el build fallaba con "Error in
loading config: La variable GOOGLE_CLIENT_ID es obligatoria" -- mismo
patron ya usado para JWT_SECRET/MEILISEARCH_API_KEY.

workloads/commerce-backend/Dockerfile

@@ -23,7 +23,10 @@ ENV CI=true \
     S3_BUCKET=ari-shopping \
     S3_ENDPOINT=http://127.0.0.1:9000 \
     MEILISEARCH_HOST=http://127.0.0.1:7700 \
-    MEILISEARCH_API_KEY=build-only-meili-key
+    MEILISEARCH_API_KEY=build-only-meili-key \
+    GOOGLE_CLIENT_ID=build-only-google-client-id \
+    GOOGLE_CLIENT_SECRET=build-only-google-client-secret \
+    GOOGLE_CALLBACK_URL=http://localhost:9000/auth/customer/google/callback
 
 COPY package.json package-lock.json .npmrc ./