feat: check-docker-composer-platform action to test docker image

.github/workflows/ci.yaml

@@ -21,7 +21,7 @@ jobs:
       # Github doesn't seem to like a working-directory arg for a custom action
       # and I ideally want to test this works as expected in the root
       - name: Copy test files to root
-        run:  cp test/cached-composer-install/composer.* .
+        run: cp test/cached-composer-install/composer.* .
 
       - name: Install without dev dependencies
         uses: ./cached-composer-install
@@ -40,3 +40,36 @@ jobs:
         env:
           EXPECT_DEV_PACKAGES: installed
         run: php test/cached-composer-install/check-installed.php
+
+  integration-test-check-docker-composer-platform:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - uses: shivammathur/setup-php@v2
+        with:
+          php-version: 8.3 # note, using 8.3 to prove that the composer reqs are only met within the mounted image
+          tools: composer:v2
+
+      - name: Pass with correct php version
+        uses: ./check-docker-composer-platform
+        with:
+          image: "php:8.4-alpine"
+          composer_json_dir: '${{ github.workspace }}/test/check-docker-composer-platform'
+
+      - name: Fail with incorrect php version
+        id: with-wrong-version
+        continue-on-error: true
+        uses: ./check-docker-composer-platform
+        with:
+          image: "php:8.3-alpine"
+          composer_json_dir: '${{ github.workspace }}/test/check-docker-composer-platform'
+
+      - name: Assert failed steps should have failed
+        shell: bash
+        run: |
+          if [ "${{ steps.with-wrong-version.outcome }}" != "failure" ] ; then
+            echo "Should have failed with incorrect PHP version in image, got ${{ steps.with-wrong-version.outcome }}"
+            exit 1
+          fi

check-docker-composer-platform/action.yml

@@ -0,0 +1,39 @@
+name: 'Check docker image meets composer platform requirements'
+description: 'Verifies that the PHP and extension versions in a docker image meet the platform requirements of the project and any composer-managed dependencies'
+inputs:
+  image:
+    description: 'The docker image name and tag to check'
+    required: true
+  composer_json_dir:
+    description: 'The directory holding the composer.json and composer.lock - defaults to the main workspace'
+    default: ${{ github.workspace }}
+runs:
+  using: 'composite'
+  steps:
+    - id: find_composer
+      name: Identify path to composer binary
+      shell: bash
+      run: |
+        set +e errexit
+        composer_bin=$(which composer)
+        set -e errexit
+        if [ -z "$composer_bin" ] ; then
+          echo "Cannot find composer binary on the github actions runner: have you run the setup-php action?"
+          exit 1
+        fi
+
+        echo "composer_bin=$composer_bin" >> $GITHUB_OUTPUT
+
+    - name: Run composer check-platform-reqs
+      shell: bash
+      run: |
+        docker run \
+        --rm \
+        --quiet \
+        -v "${{ steps.find_composer.outputs.composer_bin }}":/usr/bin/composer \
+        -v "${{ inputs.composer_json_dir }}":/workspace \
+        -w /workspace \
+        --entrypoint /usr/bin/composer \
+        "${{ inputs.image }}" \
+        check-platform-reqs \
+        --lock

test/check-docker-composer-platform/composer.json

@@ -0,0 +1,5 @@
+{
+    "require": {
+        "php": "8.4.*"
+    }
+}