Design: Test TinyPilot Debian package install

The TinyPilot install process is sufficiently complex that it would be helpful to have tests validating that the installer does what we expect.

We've had a few bugs in the past due to subtle oversights in install logic, and we only discovered them when they bit us later.

• https://github.com/tiny-pilot/tinypilot/pull/1617
• https://github.com/tiny-pilot/tinypilot/pull/1568

Rough idea

1. Create a Docker container that emulates Raspberry Pi OS Lite (put in paths like /boot/config.txt that exist in Pi OS)
   • This will probably just be the standard Debian:11 image with a few changes on top to put in paths that make it look like Pi OS as far as TinyPilot is concerned.
2. Boot into the Docker container in CircleCI
3. Install the TinyPilot Debian package (or run the bundle install script)
4. Run checks to verify that the system is in the expected state (e.g., our desired modifications to /boot/config.txt are there, paths we want to create exist, users exist that we expect).
5. Uninstall the TinyPilot Debian package
6. Reinstall the TinyPilot Debian package
7. Verify that modifications to the files we expect happen exactly once rather than once per install

Deliverable

This ticket is just to flesh out the design rather than to get into implementation.

The design should cover:

• What tests will we perform to validate the install?
• How can we create a Docker container that mimics Raspberry Pi OS for our purposes?

I accidentally duplicated this idea in https://github.com/tiny-pilot/tinypilot/issues/1691 so there are additional ideas there.

Start by reading bundler/bundle/install and debian-pkg/debian/tinypilot.postinst, then review the related ideas in issue 1691. Define the installation and reinstallation checks, plus how a Docker image can mimic the required Raspberry Pi OS paths. Done means a documented design addressing both deliverables, not an implementation.