Add specific test for a real gocd-agent-docker-dind usage

[chadlwilson]

There have been a couple of incidents in the last year where something has changed within the base docker:dind image that has caused our agent image to break when actually used with a docker-in-docker or docker-out-of-docker workflow.

In both cases this could have been caught if the functional test that validates the dind image ran a simple task such as docker run hello-world (or arguably a simple docker build) which actually interacts with the domain and/or docker sock mounted.

Although this would be docker-in-docker-in-docker, I believe this should work OK given we can share the host socker all the way through from ECS VM -> build.gocd.org centos agent container -> gocd-agent-docker-dind container -> docker run hello-world container

#11378
#12765