ci: generate multiarch manifest for bookworm

Since we're building multiple architectures to separate tags, build a
manifest to make them all available under fluxrm/flux-core:bookworm

To avoid a race condition between pushing the amd64 image, normally
called flux-core:bookworm, and pushing the manifest, also named
flux-core:bookworm, add platform to the matrix entry to force it to be
called bookworm-amd64.  The manifest will then make all of 386, arm64
and amd64 images available under the name flux-core:bookworm.

.github/workflows/main.yml

@@ -195,3 +195,21 @@ jobs:
         files: flux-core*.tar.gz
         body: |
           View [Release Notes](https://github.com/${{ github.repository }}/blob/${{ matrix.tag }}/NEWS.md) for flux-core ${{ matrix.tag }}
+
+  generate-manifest:
+    name: Generate docker manifest
+    runs-on: ubuntu-latest
+    needs: [ci-checks]
+    env:
+      DOCKER_REPO: fluxrm/flux-core
+      DOCKER_USERNAME: travisflux
+      DOCKER_PASSWORD: ${{ secrets.DOCKER_HUB_TRAVISFLUX_TOKEN }}
+      GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+    steps:
+    - name: make and push manifest as fluxrm/flux-core
+      if: >
+        (startsWith(github.ref, 'refs/tags/') || github.ref == 'refs/heads/master')
+      run: |
+        docker manifest create fluxrm/flux-core:bookworm fluxrm/flux-core:bookworm-amd64 fluxrm/flux-core:bookworm-386 fluxrm/flux-core:bookworm-arm64
+        docker manifest push fluxrm/flux-core:bookworm
+

src/test/generate-matrix.py

@@ -210,6 +210,7 @@ def __str__(self):
     env=dict(
         TEST_INSTALL="t",
     ),
+    platform="linux/amd64",
     docker_tag=True,
 )