[Makefile] Support docker-like OCI engines

[signed-log]

What does this PR do?

Remove the hard-coding for docker and allow other OCI engines to be used for testing, but keeping a hard-coded default of docker

Previous Behavior

Short of installing docker or symlinking podman (for ex.) to docker, you can't use something like podman.

New Behavior

• Create OCI variable to override the docker engine
• Refactor the test-docker error message to expose this possibility

Have tests been written for this change?

No

Have these commits been signed with GnuPG?

Yes

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