Update first step of Docker installation instructions to include download of source code
Update first step of Docker installation instructions to include download of source code

bgaleotti/proophessor-do,sandrokeil/proophessor-do,bgaleotti/proophessor-do,sandrokeil/proophessor-do,sandrokeil/proophessor-do,prooph/proophessor-do,sandrokeil/proophessor-do,prooph/proophessor-do,bgaleotti/proophessor-do,prooph/proophessor-do,bgaleotti/proophessor-do,prooph/proophessor-do