Add testflinger-testenv container

[plars]

Description

When running the test phase for agents, we currently have it run under a docker image pulled from dockerhub at plars/testflinger-testenv-focal (don't be too bothered that it's focal based, it's just to give some working env on the agent-host and this one is well tested).

Now that we're on GH for everything, we should just use ghcr.io, so that nobody has to switch it over to a new location when that bus hits me some day :)

This is the first step in resolving this. I intentionally didn't change the path pointed to by the tf-test script that the agent calls yet, because we need to make sure firewall rules are all adjusted and everything before rolling this out in production.

Resolved issues

N/A

Documentation

There's a small README there just saying what it is, but this doesn't really do anything different, just moves the location

Web service API changes

N/A

Tests

Successful action run: https://github.com/canonical/testflinger/actions/runs/11961073449
Published container image: https://github.com/canonical/testflinger/pkgs/container/testflinger%2Ftestflinger-testenv

I also tried pulling this container image and running it locally.

[plars]

A few more changes:

• Added some packages that George requested
• matrix build so that it makes a tagged version of this for focal, jammy, and noble. This way we could try out newer versions if we want.

[nancyc12]

Thanks for making this workflow! I only have a small question :)

[nancyc12]

LGTM+1