tests: Add a test for app layer ordering

`containertool` currently prepends the app layer to the layer stack,
but it should append it.   This means that a later layer will not
override a file with the same path in an earlier layer.

This commit adds an integration tests which stacks two layers on
top of each other using containertool and verifies that the second
layer overrides the first.

Author: euanh

.github/workflows/interop_tests.yml

@@ -0,0 +1,41 @@
+name: Interop tests
+
+on:
+    workflow_call:
+#        inputs:
+#            example:
+#                required: true
+#                type: string
+
+jobs:
+    layering-test:
+        name: Layering test
+        runs-on: ubuntu-latest
+        services:
+            registry:
+                image: registry:2
+                ports:
+                    - 5000:5000
+        steps:
+            - name: Checkout repository
+              uses: actions/checkout@v4
+              with:
+                  persist-credentials: false
+
+            - name: Mark the workspace as safe
+              # https://github.com/actions/checkout/issues/766
+              run: git config --global --add safe.directory ${GITHUB_WORKSPACE}
+
+            # First layer: payload does not have to be an executable, it just has to have known contents
+            - name: Build first layer
+              run: |
+                  echo first > payload
+                  swift run containertool --repository localhost:5000/layering_test payload
+                  docker run --pull=always --rm --entrypoint=cat localhost:5000/layering_test payload | grep first
+
+            # Second layer: payload does not have to be an executable, it just has to have known contents.   It should replace the first layer.
+            - name: Build another layer, which should override 'payload' from the first layer
+              run: |
+                  echo second > payload
+                  swift run containertool --repository localhost:5000/layering_test payload --from localhost:5000/layering_test:latest
+                  docker run --pull=always --rm --entrypoint=cat localhost:5000/layering_test payload | grep second

.github/workflows/pull_request.yml

@@ -14,6 +14,7 @@ jobs:
             license_header_check_project_name: "SwiftContainerPlugin"
             shell_check_container_image: "swift:6.0-noble"
 
+    # Unit tests for functions and modules
     unit-tests:
         name: Unit tests
         uses: apple/swift-nio/.github/workflows/unit_tests.yml@main
@@ -24,6 +25,7 @@ jobs:
             linux_nightly_6_1_arguments_override: "--skip SmokeTests"
             linux_nightly_main_arguments_override: "--skip SmokeTests"
 
+    # Test functions and modules against an separate registry
     integration-tests:
         name: Integration tests
         runs-on: ubuntu-latest
@@ -51,6 +53,12 @@ jobs:
               run: |
                   swift test
 
+    # Test that outputs can be handled properly by other systems
+    interop-tests:
+        name: Interop tests
+        uses: ./.github/workflows/interop_tests.yml
+
+    # Full build-package-deploy-run cycles
     endtoend-tests:
         name: End to end tests
         uses: ./.github/workflows/endtoend_tests.yml