🧪 Use prefetched action to make trampoline

action.yml

@@ -130,24 +130,28 @@ runs:
       PR_REF: ${{ github.event.pull_request.head.ref }}
       PR_REPO: ${{ github.event.pull_request.head.repo.full_name }}
       PR_REPO_ID: ${{ github.event.pull_request.base.repo.id }}
-  - name: Check out action repo
-    uses: actions/checkout@v4
-    with:
-      path: action-repo
-      ref: ${{ steps.set-repo-and-ref.outputs.ref }}
-      repository: ${{ steps.set-repo-and-ref.outputs.repo }}
   - name: Create Docker container action
     run: |
       # Create Docker container action
-      python create-docker-action.py
+      python ${{ github.action_path }}/create-docker-action.py
     env:
       REF: ${{ steps.set-repo-and-ref.outputs.ref }}
       REPO: ${{ steps.set-repo-and-ref.outputs.repo }}
       REPO_ID: ${{ steps.set-repo-and-ref.outputs.repo-id }}
     shell: bash
-    working-directory: action-repo
   - name: Run Docker container
-    uses: ./action-repo/.github/actions/run-docker-container
+    # The generated trampoline action must exist in the allowlisted
+    # runner-defined working directory so it can be referenced by the
+    # relative path starting with `./`.
+    #
+    # This mutates the end-user's workspace slightly but uses a path
+    # that is unlikely to clash with somebody else's use.
+    #
+    # We cannot use randomized paths because the composite action
+    # syntax does not allow accessing variables in `uses:`. This
+    # means that we end up having to hardcode this path both here and
+    # in `create-docker-action.py`.
+    uses: ./.github/.tmp/.generated-actions/run-pypi-publish-in-docker-container
     with:
       user: ${{ inputs.user }}
       password: ${{ inputs.password }}

create-docker-action.py

@@ -10,10 +10,12 @@
 REPO_ID = os.environ['REPO_ID']
 REPO_ID_GH_ACTION = '178055147'
 
+ACTION_SHELL_CHECKOUT_PATH = pathlib.Path(__file__).parent.resolve()
+
 
 def set_image(ref: str, repo: str, repo_id: str) -> str:
     if repo_id == REPO_ID_GH_ACTION:
-        return '../../../Dockerfile'
+        return str(ACTION_SHELL_CHECKOUT_PATH / 'Dockerfile')
     docker_ref = ref.replace('/', '-')
     return f'docker://ghcr.io/{repo}:{docker_ref}'
 
@@ -70,6 +72,20 @@ def set_image(ref: str, repo: str, repo_id: str) -> str:
     },
 }
 
-action_path = pathlib.Path('.github/actions/run-docker-container/action.yml')
+# The generated trampoline action must exist in the allowlisted
+# runner-defined working directory so it can be referenced by the
+# relative path starting with `./`.
+#
+# This mutates the end-user's workspace slightly but uses a path
+# that is unlikely to clash with somebody else's use.
+#
+# We cannot use randomized paths because the composite action
+# syntax does not allow accessing variables in `uses:`. This
+# means that we end up having to hardcode this path both here and
+# in `action.yml`.
+action_path = pathlib.Path(
+    '.github/.tmp/.generated-actions/'
+    'run-pypi-publish-in-docker-container/action.yml',
+)
 action_path.parent.mkdir(parents=True, exist_ok=True)
 action_path.write_text(json.dumps(action, ensure_ascii=False), encoding='utf-8')