fix: merging digests with actions/upload-artifact@v4

v4 doesn't allow uploading to the same entry multiple times, so you need
to upload to separate ones and then merge them manually. In this case we
just glob and download all artifacts matching the pattern (as
recommended in the documentation).

To be able to use the matrix value in the artifact name we needed to
remove the slash.

.github/workflows/docker.yml

@@ -54,8 +54,8 @@ jobs:
       fail-fast: false
       matrix:
         platform:
-          - linux/amd64
-          - linux/arm64
+          - amd64
+          - arm64
     permissions:
       contents: read
       packages: write
@@ -102,7 +102,7 @@ jobs:
           context: .
           push: ${{ inputs.push }}
           labels: ${{ steps.meta.outputs.labels }}
-          platforms: ${{ matrix.platform }}
+          platforms: linux/${{ matrix.platform }}
           build-args: ${{ inputs.build_args }}
           cache-from: type=gha
           cache-to: type=gha,mode=max
@@ -123,7 +123,7 @@ jobs:
       - name: Upload digest
         uses: actions/upload-artifact@v4
         with:
-          name: digests-${{ inputs.registry }}-${{ inputs.image_name }}-${{ steps.tag.outputs.tags }}
+          name: digests-${{ inputs.registry }}-${{ inputs.image_name }}-${{ steps.tag.outputs.tags }}-${{ matrix.platform }}
           path: /tmp/digests/*
           if-no-files-found: error
           retention-days: 1
@@ -138,8 +138,9 @@ jobs:
       - name: Download digests
         uses: actions/download-artifact@v4
         with:
-          name: digests-${{ inputs.registry }}-${{ inputs.image_name }}-${{ needs.docker.outputs.tags }}
           path: /tmp/digests
+          pattern: digests-${{ inputs.registry }}-${{ inputs.image_name }}-${{ needs.docker.outputs.tags }}-*
+          merge-multiple: true
 
       - name: Set up Docker Buildx
         uses: famedly/setup-buildx-action@v2