Update Docker installation instructions for macOS (#5725)
Update Docker installation instructions for macOS (#5725)

CONTRIBUTING.md contained an out-of-date brew command, using `brew cask` instead of `brew install --cask`
erkyrath/linguist,github/linguist,github/linguist,erkyrath/linguist,erkyrath/linguist,github/linguist,github/linguist,erkyrath/linguist