🧪 Use prefetched action to make trampoline

Previously, the action repository was being cloned from the remote
twice, unnecessarily. This patch eliminates this step and
uses the copy that was checked out on job start.

The generated trampoline action is still copied into the allowlisted
working directory so it can be referenced by the relative path
starting with `./`.

It is now output under
`./.github/.tmp/.generated-actions/run-pypi-publish-in-docker-container`
which mutates the end-user's workspace slightly but uses a path that
is unlikely to clash with somebody else's use.

Unfortunately, we cannot use randomized paths because the composite
action syntax does not allow accessing variables in `uses:`.

Fixes #292.

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')