code: func getShell(c *container.Config, os string) []string {
  	if 0 == len(c.Shell) {
  		return append([]string{}, defaultShellForOS(os)[:]...)
  	}
  	return append([]string{}, c.Shell[:]...)
  }
docs: // getShell is a helper function which gets the right shell for prefixing the
  // shell-form of RUN, ENTRYPOINT and CMD instructions