Building and pushing images with tptdev only supports dockerhub

[lander2k2]

This function is used by tptdev to push docker images: https://github.com/threeport/threeport/blob/0.6/pkg/threeport-installer/v0/tptdev/images.go#L186

It is called from the build command here: https://github.com/threeport/threeport/blob/0.6/cmd/tptdev/cmd/build.go#L114

It currently only supports pushing to dockerhub. We should add support for pushing to a local registry as well.