fix: use env vars for docker registry credentials

Pass FORGEJO_REGISTRY_USER and FORGEJO_REGISTRY_TOKEN via environment
variables instead of direct secret interpolation. This is the standard
approach used across all repos and prevents credentials from being exposed
in logs or shell history.

Fixes registry login failures by using the proven pattern from riotpiao.com.

.gitea/workflows/ci.yaml

@@ -54,21 +54,13 @@ jobs:
           SHORT_SHA=$(git rev-parse --short HEAD)
           echo "short_sha=${SHORT_SHA}" >> $GITHUB_OUTPUT
 
-      - name: Validate registry credentials
-        run: |
-          if [ -z "${{ secrets.FORGEJO_REGISTRY_USER }}" ] || [ -z "${{ secrets.FORGEJO_REGISTRY_TOKEN }}" ]; then
-            echo "❌ ERROR: Registry secrets not configured"
-            echo "Set FORGEJO_REGISTRY_USER and FORGEJO_REGISTRY_TOKEN in repo settings"
-            exit 1
-          fi
-          echo "✓ Registry credentials configured"
-
       - name: Registry login
         run: |
-          echo "${{ secrets.FORGEJO_REGISTRY_TOKEN }}" | docker login "${{ env.REGISTRY }}" \
-            --username "${{ secrets.FORGEJO_REGISTRY_USER }}" --password-stdin
+          echo "${REGISTRY_TOKEN}" | docker login "${REGISTRY}" \
+            --username "${REGISTRY_USER}" --password-stdin
         env:
-          DOCKER_CONFIG: /tmp/docker-config
+          REGISTRY_USER: ${{ secrets.FORGEJO_REGISTRY_USER }}
+          REGISTRY_TOKEN: ${{ secrets.FORGEJO_REGISTRY_TOKEN }}
 
       - name: Build Docker image
         run: |