Tests should support local and remote daemons about imgutil

[micahyoung]

Running against a remote daemon works now thanks to docker_registry.go, so long as your daemon.json has an insecure-registries entry with a <host ip>/32 set, where <host ip> is your DOCKER_HOST=tcp://<host ip>.

Example:

Remote host's daemon.json (assuming it has IP 192.168.86.5)

{
  "insecure-registries": [
    "192.168.86.5/32"
  ],
  "hosts": ["<configured for remote access>"]
}

Run tests on local workstation:

export DOCKER_HOST=tcp://192.168.86.5:2375
make test

@ekcasey I think this can be closed as most of the other behavior you describe is implemented and if this setup is acceptable?

[micahyoung]

This is technically a nice-to-have for the Windows work, since we could for now just run imgutil tests on Windows workstation targetting a local Docker Desktop with Windows containers.

[ekcasey]

I agree that running against a remote docker daemon should be supported.

For the remote tests, I wonder if the registry should be configurable, rather than deployed to the docker daemon as part of the test code? The repository could be supplied to the tests as an environment variable, and test images could be generated with randomized tags that are cleaned up afterwards. Credentials would need to be set prior to running the tests either in the default docker config json or in a docker config set with DOCKER_CONFIG (I believe both of these would work with the authn.DefaultKeychain).

Our CI scripts could deploy a registry to the daemon and set the credentials before running the tests.