Fix invalid Docker tag format in GitHub Actions workflow

Changed SHA tag prefix from `{{branch}}-` to `sha-` to fix build
failures when creating releases from tags. The `{{branch}}` variable
is empty when building from tags (e.g., v1.0.0), which resulted in
invalid Docker tags like `:-7bdde95` instead of `sha-7bdde95`.

This fixes the v1.0.0 release build failure.

Generated with [Claude Code](https://claude.ai/code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

.github/workflows/build-and-push.yaml

@@ -49,7 +49,7 @@ jobs:
             type=semver,pattern={{version}}
             type=semver,pattern={{major}}.{{minor}}
             type=semver,pattern={{major}}
-            type=sha,prefix={{branch}}-
+            type=sha,prefix=sha-
             type=raw,value=latest,enable={{is_default_branch}}
 
       - name: Build and push Docker image