Some tweaks for use in gradle-docker.

- Change the default Docker port to 2375 to match current Docker version.

- Add toString() to all commands which prints out a form similar to the
  corresponding Docker command line.  Useful in tracing.

- Create client logging filter variant which allows us to avoid logging
  requests whose content type is likely to cause issues for both the
  Windows and OSX consoles.  This is an adaptation of a change from a
  while back
  (https://github.com/alexec/docker-java/commit/887a1120e5d3c54afa51e84f360ca6740d9916ec)
  which seems to have be lost in the move.