[CI] sgl-kernel: prune dangling images before each wheel build (#23723)

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.github/workflows/pr-test.yml

@@ -479,6 +479,26 @@ jobs:
         with:
           python-version: ${{ matrix.python-version }}
 
+      - name: Free Docker disk space
+        run: |
+          set -x
+          # build.sh retags sgl-kernel-deps:cuda${CUDA_VERSION}-${PY_TAG}-${ARCH}
+          # on every run, leaving the previous image as a dangling <none>:<none>
+          # entry (~16-23 GB each). Prune them before building so the runner
+          # doesn't fill up. The local buildx cache at ~/.cache/sgl-kernel/buildx
+          # and the tagged sgl-kernel-deps image are not affected.
+          # `until=12h` avoids racing with a sibling matrix cell (cuda 12.9 vs
+          # 13.0) that may have just orphaned an image seconds ago.
+          docker image prune -f --filter "until=12h"
+          # Drop orphaned buildx builder volumes from past `docker buildx create`
+          # invocations. The active `sgl-kernel-builder` volume is held open and
+          # would fail to remove anyway, but skip it explicitly for clarity.
+          for v in $(docker volume ls -q | grep '^buildx_buildkit_' | grep -v '^buildx_buildkit_sgl-kernel-builder' || true); do
+            echo "Removing orphan buildx volume: $v"
+            docker volume rm "$v" || true
+          done
+          df -h /
+
       - name: Build wheel for Python ${{ matrix.python-version }} and CUDA ${{ matrix.cuda-version }}
         run: |
           cd sgl-kernel
@@ -545,6 +565,17 @@ jobs:
         with:
           python-version: ${{ matrix.python-version }}
 
+      - name: Free Docker disk space
+        run: |
+          set -x
+          # See sgl-kernel-build-wheels above for the rationale.
+          docker image prune -f --filter "until=12h"
+          for v in $(docker volume ls -q | grep '^buildx_buildkit_' | grep -v '^buildx_buildkit_sgl-kernel-builder' || true); do
+            echo "Removing orphan buildx volume: $v"
+            docker volume rm "$v" || true
+          done
+          df -h /
+
       - name: Build wheel for Python ${{ matrix.python-version }} and CUDA ${{ matrix.cuda-version }}
         run: |
           cd sgl-kernel