Fix Docker tag: sanitize ref_name to replace / with -

On PR events github.ref_name is '142/merge' — slashes are invalid in
Docker image tags. Replace / with - before using as a tag.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Committed-By-Agent: claude

Author: tonyxiao

.github/workflows/build.yml

@@ -51,20 +51,24 @@ jobs:
           username: ${{ github.actor }}
           password: ${{ secrets.GITHUB_TOKEN }}
 
+      - name: Sanitize Docker tag
+        id: tag
+        run: echo "value=$(echo '${{ github.ref_name }}' | sed 's/\//-/g')" >> $GITHUB_OUTPUT
+
       - name: Build Docker image
         run: |
           docker build \
             --build-arg GIT_COMMIT=${{ github.sha }} \
             --build-arg BUILD_DATE=$(date -u +%Y-%m-%dT%H:%M:%SZ) \
             --build-arg COMMIT_URL=${{ github.server_url }}/${{ github.repository }}/commit/${{ github.sha }} \
             -t ghcr.io/${{ github.repository }}:${{ github.sha }} \
-            -t ghcr.io/${{ github.repository }}:${{ github.ref_name }} \
+            -t ghcr.io/${{ github.repository }}:${{ steps.tag.outputs.value }} \
             .
 
       - name: Push Docker image
         run: |
           docker push ghcr.io/${{ github.repository }}:${{ github.sha }}
-          docker push ghcr.io/${{ github.repository }}:${{ github.ref_name }}
+          docker push ghcr.io/${{ github.repository }}:${{ steps.tag.outputs.value }}
 
       - name: Docker smoke test
         run: docker run --rm ghcr.io/${{ github.repository }}:${{ github.sha }} --version