ENH: Convert from Docker action to composite action

Replace the Docker-based action (FROM ubuntu:18.04) with a composite
action that runs directly on the GitHub Actions runner. This fixes
two recurring CI reliability issues:

1. Docker Hub rate limiting (401/429) pulling ubuntu:18.04 (EOL)
   which intermittently breaks the linter across all remote modules.

2. Transient pixi download failures (HTTP 502) from the raw
   curl-pipe-to-bash install method.

The composite action uses:
- prefix-dev/setup-pixi@v0.9.4 for reliable pixi installation with
  built-in caching and retry
- curl --retry for resilient downloads of ITK configuration files
- Direct runner execution (no Docker pull needed)

The Dockerfile and entrypoint.sh are retained for backwards
compatibility but are no longer referenced by action.yml.

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

Author: hjmjohnson

action.yml

@@ -9,8 +9,72 @@ inputs:
     description: 'The Git branch of the ITK repository to fetch .clang-format from.'
     default: 'main'
 runs:
-  using: 'docker'
-  image: 'Dockerfile'
-  args:
-    - ${{ inputs.error-message }}
-    - ${{ inputs.itk-branch }}
+  using: 'composite'
+  steps:
+    - name: Fetch ITK clang-format configuration
+      shell: bash
+      run: |
+        set -euo pipefail
+        ITK_BRANCH="${{ inputs.itk-branch }}"
+
+        # Fetch the .clang-format file from the specified branch
+        if ! test -f ./.clang-format; then
+          echo "Downloading ITK .clang-format from ${ITK_BRANCH}"
+          curl --retry 3 --retry-delay 10 --retry-all-errors -fsSL \
+            "https://raw.githubusercontent.com/InsightSoftwareConsortium/ITK/${ITK_BRANCH}/.clang-format" \
+            -o .clang-format
+        fi
+
+        # Fetch the clang-format runner script
+        curl --retry 3 --retry-delay 10 --retry-all-errors -fsSL \
+          "https://raw.githubusercontent.com/InsightSoftwareConsortium/ITK/${ITK_BRANCH}/Utilities/Maintenance/clang-format.bash" \
+          -o clang-format.bash
+        chmod +x ./clang-format.bash
+
+        # Determine clang-format version from ITK pre-commit config
+        if test "${ITK_BRANCH}" != "release-5.4" -a "${ITK_BRANCH}" != "release"; then
+          curl --retry 3 --retry-delay 10 --retry-all-errors -fsSL \
+            "https://raw.githubusercontent.com/InsightSoftwareConsortium/ITK/${ITK_BRANCH}/.pre-commit-config.yaml" \
+            -o ITK.pre-commit-config.yaml
+          CLANG_FORMAT_VERSION=$(grep -A 1 "mirrors-clang-format" ITK.pre-commit-config.yaml | tail -n 1 | cut -d: -f2 | tr -d ' v')
+          echo "CLANG_FORMAT_VERSION=${CLANG_FORMAT_VERSION}" >> $GITHUB_ENV
+        fi
+
+    - name: Install pixi
+      if: env.CLANG_FORMAT_VERSION != ''
+      uses: prefix-dev/setup-pixi@v0.9.4
+
+    - name: Install clang-format via pixi
+      if: env.CLANG_FORMAT_VERSION != ''
+      shell: bash
+      run: |
+        pixi global install clang-format==${{ env.CLANG_FORMAT_VERSION }}
+        echo "$HOME/.pixi/bin" >> $GITHUB_PATH
+        # Find the installed clang-format and add to PATH
+        PIXI_ENV_PATH=$(pixi global list 2>/dev/null | grep -o '/[^ ]*envs/clang-format' || echo "")
+        if [ -n "${PIXI_ENV_PATH}" ]; then
+          echo "${PIXI_ENV_PATH}/bin" >> $GITHUB_PATH
+        fi
+        # Also check common pixi global install locations
+        for p in "$HOME/.pixi/envs/clang-format/bin" "$HOME/.pixi/envs/default/bin"; do
+          [ -d "$p" ] && echo "$p" >> $GITHUB_PATH
+        done
+
+    - name: Run clang-format check
+      shell: bash
+      run: |
+        set -euo pipefail
+        ./clang-format.bash --tracked
+        if ! git diff-index --diff-filter=M --quiet HEAD -- ':!.clang-format'; then
+          echo "::error ::${{ inputs.error-message }}"
+          echo ""
+          echo "Changes required:"
+          echo ""
+          echo "Files:"
+          git diff-index --diff-filter=M --name-only HEAD -- ':!.clang-format'
+          echo ""
+          echo "Changes:"
+          git diff HEAD -- ':!.clang-format'
+          exit 1
+        fi
+        echo "clang-format ITK Coding Style check completed successfully."