containertool: Use the default registry parameter for image uploads

[euanh]

Motivation

The --default-registry parameter (also set by the CONTAINERTOOL_DEFAULT_REGISTRY environment variable) allows a default registry address to be specified for unqualified image names. For example, the following uploads should be equivalent:

containertool --default-registry myregistry.com --repository myrepo/myimage --from baserepo/baseimage
containertool --repository myregistry.com/myrepo/myimage --from baserepo/baseimage

However the default registry was only being applied when fetching the base image (the --from parameter) and not when pushing the final image (the --repository parameter).

Modifications

The default registry parameter is now used when pushing the final image.

Result

Pushes to unqualified repositories will work properly.

Test Plan

Manually tested containertool uploads with a variety of qualified and unqualified image names.