CML v0.3.8

• CLI: enforce strict argument parsing (#493)
• Docker: update tags & refactor build (#494)
  • tag convention: dvgorg/cml:0-dvc{1,2}-base{0,1} (#217)
  • py3.8: base1 (#360)
  • add alternate registry: ghcr.io (#500 <- #267)
• misc documentation updates
  • REPO_TOKEN (deprecate repo_token) (#492)
  • docker tags (#499)
• misc test updates (#498)
  • fix bot tests
  • fix potential security issues
• misc tidy (#498)
  • lint YAML (#497)