code: func dispatchShell(d dispatchRequest, c *instructions.ShellCommand) error {
  	d.state.runConfig.Shell = c.Shell
  	return d.builder.commit(d.state, fmt.Sprintf("SHELL %v", d.state.runConfig.Shell))
  }
docs: // SHELL powershell -command
  //
  // Set the non-default shell to use.