Skip cleaning up the default docker image to avoid redundant pulls

The default image (clang-20) is reused across multiple steps within
the same job. Only remove non-default images after use.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hsbt

.github/actions/compilers/action.yml

@@ -153,9 +153,12 @@ runs:
         GITHUB_PULL_REQUEST_URL: ${{ github.event.pull_request.html_url }}
         GITHUB_REF: ${{ github.ref }}
 
+    # Clean up non-default docker images to save disk space.
+    # The default image (clang-20) is reused across multiple steps
+    # within the same job, so we keep it to avoid redundant pulls.
     - name: clean up docker image
       shell: bash
       run: docker rmi "ghcr.io/ruby/ruby-ci-image:${INPUT_TAG}" || true
-      if: always()
+      if: ${{ always() && inputs.tag != 'clang-20' }}
       env:
         INPUT_TAG: ${{ inputs.tag }}