Add a docker manifest test workflow

.github/workflows/test-docker-manifest.yaml

@@ -0,0 +1,29 @@
+name: Upload release artifacts
+on:
+  workflow_dispatch:
+    inputs:
+      release_tag:
+        description: "The release tag to target"
+
+permissions:
+  id-token: write
+  contents: write
+  packages: write
+
+jobs:
+  docker-multiplatform-image:
+    name: "Assemble multi-platform Docker image"
+    runs-on: ubuntu-latest
+    steps:
+      - name: Log in to registry
+        run: echo "${{ secrets.GH_TOKEN_FOR_UPDATES }}" | docker login ghcr.io -u $ --password-stdin
+      - name: Assemble and push image
+        env:
+          RELEASE_TAG: ${{ github.event_name == 'release' && github.event.release.tag_name || github.event.inputs.release_tag }}
+        run: |
+          docker manifest create \
+            ghcr.io/tweag/nickel:$RELEASE_TAG \
+            ghcr.io/tweag/nickel:$RELEASE_TAG-x86_64 \
+            ghcr.io/tweag/nickel:$RELEASE_TAG-arm64 \
+          docker manifest push --verbose ghcr.io/tweag/nickel:$RELEASE_TAG
+