Use appropriate user in CI to avoid permissions errors on later runs

[connorjward]

This should fix the permissions issues that we saw building Docker images on CI.

The issue was that the regular CI runs are run in a Docker container as the root user and this means that if a job is cancelled then a bunch of files are left behind that are owned by root. If we subsequently build a Docker image on the same runner we would crash because that workflow is run as an unprivileged user.

I tried to make the regular CI runs use an unprivileged user but I don't think that it is possible. Instead the solution I've found here is to build the Docker images inside another container, and hence as root.

[connorjward]

@JHopeCollins don't bother reviewing. Needs more thought.

[connorjward]

Suggested change

	# UNDO ME
	docker:
	name: Build developer Docker containers
	uses: ./.github/workflows/docker.yml
	with:
	tag: connorjwardtest-dev-${{ github.base_ref }}
	branch: ${{ github.base_ref }}
	build_dev: true
	secrets: inherit

[connorjward]

@JHopeCollins I think that this is ready to go now. If you have time please give this a review. The changes look daunting but the core change is we now build the Linux containers inside an ubuntu:latest container instead of directly on the runner.