feat(core): support TESTCONTAINERS_HUB_IMAGE_NAME_PREFIX (#961)

fixes #808

partially resolve #562, there may be some additional features requested
there that this commit does not resolve.

Additionally added test cases to check that it's picked up from env into
config, and that the config changes the resolved image in
DockerContainer

Co-authored-by: David Ankin <daveankin@gmail.com>