[WIP] harpoonctl dockerbuild

This subcommand will coördinate the common-case of taking an existing Docker container (either locally-built via Dockerfile, or remotely-pulled-in via repository), layering config files on top, building, running, and exporting to be consumed by Harpoon.
The leading slash is intentional in both cases. They're the absolute paths for mount points into the container (DST) and from the host system (SRC).