Update build environment prep documentation
Update build environment prep documentation

- Document the need for 4GB of memory to help users of Docker
  for Mac and Docker for Windows know to check the memory cap
  setting when encountering strange build failures

Resolves #3809

Signed-off-by: Robert Marshall <0465b3b4f3ae0103ad50d09ff86a3523700a2979@gitlab.com>

gitlabhq/omnibus-gitlab,gitlabhq/omnibus-gitlab,gitlabhq/omnibus-gitlab,gitlabhq/omnibus-gitlab