Thank you to @Mr-C for the answer to this one:

A variation on http://www.commonwl.org/user_guide/14-runtime/index.html is at https://github.com/common-workflow-language/common-workflow-language/blob/master/v1.0/v1.0/writable-dir-docker.cwl

class: CommandLineTool

requirements:
  - class: InlineJavascriptRequirement
  - class: InitialWorkDirRequirement
    listing:
      - entryname: empty_dir
        entry: "$({class: 'Directory', listing: []})"
        writable: true

inputs: []

outputs:
  empty_dir:
    type: Directory
    outputBinding:
      glob: empty_dir

I've found this useful for making temporary directories that can be used at later steps in the workflow.