[Snek] [keith-packard/snek] ad4078: .github: Reduce container needed for 'black' test

[Keith Packard]

  Branch: refs/heads/black-workflow
  Home:   https://github.com/keith-packard/snek
  Commit: ad4078493bb6bc1901b6552d4e30f7750de4946f
      https://github.com/keith-packard/snek/commit/ad4078493bb6bc1901b6552d4e30f7750de4946f
  Author: Keith Packard <keithp at keithp.com>
  Date:   2020-11-03 (Tue, 03 Nov 2020)

  Changed paths:
    A .github/Dockerfile-black
    M .github/workflows/black.yml

  Log Message:
  -----------
  .github: Reduce container needed for 'black' test

Signed-off-by: Keith Packard <keithp at keithp.com>