Release 7.8.2

Several usability fixes:
 
  + More detail / hints when refusing a SSH key
  + Only throw away a configuration file if it's using a different major version

Also several build fixes:

  + Run tests prior to building packages.
  + Escalate command failures when packaging (broke when `| sed` was added for prefixing).
  + Optimize Docker builds by installing Gosu before tools.