Dockerfile: remove `USER linuxbrew`.
Dockerfile: remove `USER linuxbrew`.

This defaults to `root` but we still want to keep a Linuxbrew user on
the system.

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