ci: handle rabbitmq version for push trigger (ietf-tools#10606)

Author: jennifer-richards

.github/workflows/build-mq-broker.yml

@@ -39,6 +39,15 @@ jobs:
         username: ${{ github.actor }}
         password: ${{ secrets.GITHUB_TOKEN }}
 
+    - name: Set rabbitmq version
+      id: rabbitmq-version
+      run: |
+        if [[ "${{ inputs.rabbitmq_version }}" == "" ]]; then
+          echo "RABBITMQ_VERSION=3.13-alpine" >> $GITHUB_OUTPUT
+        else
+          echo "RABBITMQ_VERSION=${{ inputs.rabbitmq_version }}" >> $GITHUB_OUTPUT
+        fi
+
     - name: Docker Build & Push
       uses: docker/build-push-action@v6
       env:
@@ -48,7 +57,7 @@ jobs:
         file: dev/mq/Dockerfile
         platforms: linux/amd64,linux/arm64
         push: true
-        build-args: RABBITMQ_VERSION=${{ inputs.rabbitmq_version }}
+        build-args: RABBITMQ_VERSION=${{ steps.rabbitmq-version.outputs.RABBITMQ_VERSION }}
         tags: |
-          ghcr.io/ietf-tools/datatracker-mq:${{ inputs.rabbitmq_version }}
+          ghcr.io/ietf-tools/datatracker-mq:${{ steps.rabbitmq-version.outputs.RABBITMQ_VERSION }}
           ghcr.io/ietf-tools/datatracker-mq:latest