added apt-get update to Dockerfile

Hi fig-Team,
I really enjoyed the rails app example, it's amazing.
But at first it didn't work, but it it was a trivial problem.
One needs to update apt-get in the Dockerfile, otherwise sometimes apt get throws some errors like in my case.

Here the pull request for this critical bug ;-)

Best regards and I hope to see more of fig in the near future!!
Sebastian