fix: Add Docker authentication for GHCR in release workflow

Add docker/setup-buildx-action and docker/login-action steps before
GoReleaser to properly authenticate with GitHub Container Registry.
This fixes the 'unauthorized: unauthenticated' error when pushing
Docker images.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: pfrederiksen

.github/workflows/release.yml

@@ -30,6 +30,16 @@ jobs:
       - name: Run tests
         run: go test -v ./...
 
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Login to GitHub Container Registry
+        uses: docker/login-action@v3
+        with:
+          registry: ghcr.io
+          username: ${{ github.repository_owner }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
       - name: Run GoReleaser
         uses: goreleaser/goreleaser-action@v6
         with: