Fix some typos and add built-tags output support

This change makes some minor changes:

- Fix some typos
- Add a "test your changes" example to the README
- Add a --nocache flag to ignore the Docker build cache
- Add a --write_tags_to flag to save a clean list of
  built tags to a file. Can be used to build tags, then
  run tests in parallel by combining xargs with --only_tags_matching.