Dockerfile: run `git fetch`.
Dockerfile: run `git fetch`.

This should setup `origin/master` correctly for
https://github.com/Homebrew/homebrew-test-bot/pull/424.

sjackman/homebrew,mahori/brew,nandub/brew,MikeMcQuaid/brew,nandub/brew,nandub/brew,Homebrew/brew,JCount/brew,claui/brew,EricFromCanada/brew,claui/brew,EricFromCanada/brew,nandub/brew,mahori/brew,sjackman/homebrew,claui/brew,vitorgalvao/brew,JCount/brew,Homebrew/brew,EricFromCanada/brew,sjackman/homebrew,claui/brew,Homebrew/brew,MikeMcQuaid/brew,JCount/brew,konqui/brew,Homebrew/brew,konqui/brew,konqui/brew,konqui/brew,vitorgalvao/brew,vitorgalvao/brew,mahori/brew,vitorgalvao/brew,MikeMcQuaid/brew,MikeMcQuaid/brew,EricFromCanada/brew,mahori/brew,JCount/brew,sjackman/homebrew,reitermarkus/brew,reitermarkus/brew,reitermarkus/brew,reitermarkus/brew