Update PS1 during wwctl image shell to include working directory

Optionally, use `WW_PS1` to construct the prompt.

- Closes: #1245

Signed-off-by: Jonathon Anderson <janderson@ciq.com>

CHANGELOG.md

@@ -25,6 +25,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 - syncuser overlay skips duplicate users and groups in passwd and group databases. #829
 - `wwctl image syncuser --write` is true by default. #1736
 - Update syncuser documentation. #1736
+- Update PS1 during `wwctl image shell` to include working directory by default,
+  and to include `PS1` from the environment if present. #1245
 
 ### Fixed