Docker: add optional test input

.github/workflows/docker.yml

@@ -30,6 +30,13 @@ on:
         required: false
         type: string
 
+      # Shell command to test the image before the push. The workflow first
+      # builds the image for linux/amd64 and loads it into Docker as
+      # IMAGE (image:tag). The command runs from the repository root.
+      test:
+        required: false
+        type: string
+
 jobs:
   docker-workflow:
     name: "${{ inputs.image }}:${{ inputs.tag }}"
@@ -54,6 +61,23 @@ jobs:
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v4
 
+      - name: Build test image
+        if: inputs.test
+        uses: docker/build-push-action@v7
+        with:
+          context: ${{ inputs.context }}
+          file: ${{ inputs.dockerfile || format('{0}/Dockerfile', inputs.context) }}
+          tags: ${{ inputs.image }}:${{ inputs.tag }}
+          platforms: linux/amd64
+          load: true
+          cache-from: type=gha
+
+      - name: Test
+        if: inputs.test
+        env:
+          IMAGE: ${{ inputs.image }}:${{ inputs.tag }}
+        run: ${{ inputs.test }}
+
       - name: Build and push
         uses: docker/build-push-action@v7
         with: