Fix the docker tests

Before this patch, our docker tests consisted of an integration test, which tested the script element, docker-source-test.bst.

This script element depended on the dockerhub-alpine.bst import element as the base element.

As this is an integration test, upon success, the docker source is stored in /tmp/integration-cache/docker. From there on, every time this test is re-run we fetch the docker source from this cache rather than dockerhub. So, if we were to change our dockerhub base element, to fetch from a fake url, the test would still pass.

Because of this, I've now explicitly marked the existing test as an integration test and also added another test which tests the fetching from dockerhub.