Action: Support running in a docker container (#2748)

see: https://github.com/actions/runner/issues/716

CHANGES.md

@@ -24,6 +24,10 @@
 
 - All upper version bounds on dependencies have been removed (#2718)
 
+### Integrations
+
+- Update GitHub action to support containerized runs (#2748)
+
 ## 21.12b0
 
 ### _Black_

action.yml

@@ -38,7 +38,7 @@ runs:
         import subprocess;
         from pathlib import Path;
 
-        MAIN_SCRIPT = Path(r'${{ github.action_path }}') / 'action' / 'main.py';
+        MAIN_SCRIPT = Path(r'${GITHUB_ACTION_PATH}') / 'action' / 'main.py';
 
         proc = subprocess.run([sys.executable, str(MAIN_SCRIPT)]);
         sys.exit(proc.returncode)