Avoid timeouts by testing with a stable image repo
Avoid timeouts by testing with a stable image repo

One of the integration tests checks that the image metadata fetch
works with a real world image repository. It was using `busybox` (at
DockerHub) as that repo; but, busybox has enough manifests that the
test can time out waiting for the fetch to complete.

To keep the running time bounded, use a repo that has a small (but >
burst) number of images already, but isn't getting new tags added to
it too fast; and, tune the rate limiting down so that it won't make
too much of an impression on the registry.

weaveworks/flux,weaveworks/flux