vantagectl/ becomes the repository root, so the module is
gitea.hostxtra.co.uk/vantage/vantage-ctl and the image is
vantage/vantage-ctl. The command keeps the name vantagectl.
Release tags become a bare v*: the vantagectl/ prefix existed to
namespace one component inside a shared repository, and unlike the
agent's agent/v* nothing reads this one programmatically.
