job-runner Windows testing

[ghickman]

We can't run tests requiring Docker on Windows in CI atm.

Also, we have minimal tooling/docs for manually running job-runner in Windows

Most of our users are on Windows, this is a problem:

options:

• tooling for Windows VM w/nested virt for linux/macos
• docker api stub to run more code on windows

[evansd]

The half-assed solution I came up with when first developing job-runner was to have the Docker client in my Windows VM talk to a Docker daemon running on the host, by forwarding the socket into the VM.

In one sense, this isn't a million miles from what actually happens on Windows, where all Docker traffic goes over a VM-Host socket, except that it's inverted: here the client runs in the VM and the daemon runs on the host, whereas usually it's the other way round.

In any case, I briefly documented it here:
DEVELOPERS.md#testing-on-windows

And here are the relevant scripts:
scripts/host-services-for-win-testing.sh
scripts/run-tests-in-windows.sh

I had to do the thing with git because I found that just mounting the checkout directory into the VM didn't work (but I can't now remember exactly what weird problem it caused).

[evansd]

There has been some forward movement on support for nested virtualisation in Github Actions which would open up the possibility of running full end-to-end Docker tests on Windows in CI. See Slack thread:
https://bennettoxford.slack.com/archives/C63UXGB8E/p1719400674225889

However this still isn't quite supported in the way we need it to be. See this comment here:
actions/runner-images#7191 (reply in thread)

Have set a reminder to check back in six months and see where things stand.