docs(hacking): document MOUNT var for local Python package overrides

Author: Onibenjo

HACKING.md

@@ -82,6 +82,17 @@ You can use the `./run` script to use Node modules from a local folder on a one-
 ./run --node-module $HOME/projects/vanilla-framework watch  # Build CSS dynamically, using a local version of vanilla-framework
 ```
 
+### Overriding Python packages
+
+If you're maintaining one of the Canonical web team's Python packages (e.g. `canonicalwebteam.flask-base`, `canonicalwebteam.templatefinder`, `canonicalwebteam.blog`, `canonicalwebteam.search`) and want to test local changes against ubuntu.com without publishing a release, pass `MOUNT=<path>` to any Python-running task. The `start`, `build`, `test-python`, and `lint-python` tasks prepend that path to `PYTHONPATH`, so Python loads modules from your local checkout instead of the installed version:
+
+```bash
+task start MOUNT=$HOME/projects/canonicalwebteam.flask-base
+task test-python MOUNT=$HOME/projects/canonicalwebteam.blog
+```
+
+`MOUNT` is opt-in — leave it unset for normal runs. CI never sets it.
+
 ## Making changes to the site
 
 Guides for making changes to the ubuntu.com codebase.