Utilize ghrc for external test images

[smlambert]

Currently when we run tests from the external group, we build the Dockerfile on the fly as part of the test run, then execute the test suite. This capability is there so that we could "quickly" test other 3rd party applications via the external_custom test target. While this issue is not suggesting we drop the ability to generate a Dockerfile, for the purpose of the set of tests currently in the external directory, we can simplify and speed up the test runs by pushing pre-built images to our ghrc adoptium-test-image directory.