feat: Allow building an image without pushing

[mikebryant]

Fixes #144

[dlorenc]

Thanks for this! What do you think about adding a mode to output the image as a tarball on disk, instead of just skipping the push. That would serve the same use-case for you (you can use /dev/null or just ignore the tarball), and is something we'll probably need anyway.

[dlorenc]

Closing in favor of the tarball approach in #178