ci: generate a generic linux snapshot image

This change pushes a copy of `*-linux-gnu` as `*-linux`.

Currently the snapshots are only pushing `*-linux-gnu` and
`*-linux-musl` images.

.github/workflows/publish-images.yml

@@ -49,8 +49,11 @@ jobs:
             puts RakeCompilerDock::Starter.container_image_name(:platform => %q(${{matrix.platform}})); \
             print "snapshot_name="; \
             puts RakeCompilerDock::Starter.container_image_name(:platform => %q(${{matrix.platform}}), :version => %q(snapshot)); \
+            if %q(${{matrix.platform}}).end_with?("-gnu"); \
+              print "generic_linux_snapshot_name="; \
+              puts RakeCompilerDock::Starter.container_image_name(:platform => %q(${{matrix.platform}}), :version => %q(snapshot)).chomp("-gnu"); \
+            end \
           ' | tee -a $GITHUB_OUTPUT
-
       - name: Build docker image
         env:
           RCD_DOCKER_BUILD: docker buildx build --cache-from=type=local,src=tmp/build-cache --cache-to=type=local,dest=tmp/build-cache-new --load
@@ -69,3 +72,6 @@ jobs:
           docker images
           docker tag ${{steps.rcd_config.outputs.image_name}} ${{steps.rcd_config.outputs.snapshot_name}}
           docker push ${{steps.rcd_config.outputs.snapshot_name}}
+      - if: ${{ steps.rcd_config.output.generic_linux_snapshot_name }}
+        run: |
+          docker tag ${{steps.rcd_config.outputs.image_name}} ${{steps.rcd_config.outputs.generic_linux_snapshot_name}}