No story: Switched pex-watcher base image

Building the pex-watcher image from scratch isn't working anymore. This is due to the CentOS7 archive repos getting moved as of July 1st (from what I can tell).

This changes the base image of pex-watcher to python:3.10-slim-bullseye which no longer requires us to install Python from source. It seems to work fine building pexes locally but we should test it on dev since it's no longer a RHEL image.

Also, the pex watcher wasn't starting for me. Setting tty: true in the docker compose fixed that.