Get testing of built Docker image working in CI

[matthewfeickert]

this seemss to be too deeeply nested docker (note it mounts docker.sock which might not be avaliable on GHA). c.f. maxheld83/ghactions#307

Originally posted by @lukasheinrich in #46 (comment)

[matthewfeickert]

Ah, as a note to myself on understanding how recast is running Docker in Docker, it is mounting the socket /var/run/docker.sock

recast-atlas/src/recastatlas/backends/docker.py

Lines 28 to 39 in 9337071

	command = [
	"docker",
	"run",
	"--rm",
	"-i",
	"-v",
	"{}:{}".format(cwd, cwd),
	"-w",
	cwd,
	"-v",
	"/var/run/docker.sock:/var/run/docker.sock",
	]

and so relying on the host to provide Docker. 👍