HACKING: Add a section about Python bridge and debugging

Explain the COCKPIT_DEBUG environment variable.

Closes #19193

HACKING.md

@@ -182,14 +182,43 @@ git submodules:
 Refer to the [testing README](test/README.md) for details on running the Cockpit
 integration tests locally.
 
-## Testing the Python bridge
+## Python bridge
 
 Cockpit currently has an experimental replacement for `cockpit-bridge`
 written in Python.  It resides in `src/cockpit` with most of its rules in
 `src/Makefile.am`.  This directory was chosen because it matches the standard
 so-called "src layout" convention for Python packages, where each package
 (`cockpit`) is a subdirectory of the `src` directory.
 
+### Running the bridge
+
+The Python bridge can be used interactively on a local machine:
+
+    PYTHONPATH=src python3 -m cockpit.bridge
+
+To make it easy to test out channels without having to write out messages
+manually `cockpit.misc.print` can be used:
+
+   PYTHONPATH=src python3 -m cockpit.misc.print open fslist1 path=/etc watch=False | PYTHONPATH=src -m cockpit.bridge
+
+These shell aliases might be useful when experimenting with the protocol:
+
+    alias cpy='PYTHONPATH=src python3 -m cockpit.bridge'
+    alias cpf='PYTHONPATH=src python3 -m cockpit.misc.print'
+
+When working with the Python bridge on test images, note that `RHEL/CentOS 8`,
+`debian-stable` and `ubuntu-2204` use the C bridge. So if you want to
+explicitly have the Python bridge on those images use:
+
+    ./test/image-prepare --python
+
+To enable debug logging in journal on a test image, you can pass `--debug` to
+`image-prepare`. This will set `COCKPIT_DEBUG=all` to `/etc/environment`, if
+you are only interested channel debug messages change `all` to
+`cockpit.channel`.
+
+### Testing the Python bridge
+
 There are a growing number of Python `unittest` tests being written to test
 parts of the new bridge code.  You can run these with `make pytest` or
 `make pytest-cov`.  Those are both just rules to make sure that the