Support HTTP when pushing with --insecure-registry

[kflansburg]

This Closes #297 by cherry picking this commit from @aler9.

My test case was being able to push to a stock Docker registry container with no configurations set (and thus using HTTP).

I suspect that this may break the use case of pushing to an HTTPS endpoint whilst ignoring a custom CA using --insecure-registry. If that is a requirement then we may need to introduce an argument to specify protocol.