message: docker: Add folder name as suffix when adding folder.
  - DockerFile.add([node('foo/bar'), node('foo/baz')], '/usr') is now
  translated to "ADD foo /usr/foo".
diff: @@ -161,7 +161,10 @@ class DockerFile(drake.Node):
  chain(*(parents(n.path().without_prefix(root))
  for n in installed_files(nodes)
  if n is not drake.Path.dot))):
  - print('ADD %s %s' % (add, p), file = f)
  + dest = "%s/%s" % (p, add) \
  + if os.path.isdir(str(drake.Path(root) / add)) \
  + else p
  + print('ADD %s %s' % (add, dest), file = f)
  for p in self.__dockerfile.ports_:
  print('EXPOSE %s' % p, file = f)
  if self.__dockerfile.volumes_:
  