ci: authenticate cluster registry push

.gitea/workflows/build.yml

@@ -12,6 +12,13 @@ jobs:
         with:
           fetch-depth: 1
 
+      - name: Authenticate to the cluster registry
+        run: |
+          printf '%s' '${{ secrets.REGISTRY_PASSWORD }}' \
+            | docker login registry.a2acloud.io \
+                --username '${{ secrets.REGISTRY_USER }}' \
+                --password-stdin
+
       - name: Build production image in Kubernetes
         run: |
           set -euo pipefail