Cleanup code in the bootstrap function

Cleanup code in the bootstrap function

I came here to do something else, but could not help cleaning up the bootstrap
code.

* add die() helper
* pull the container ID into an environment variable right away instead of
  interpolating it every time
* we don't need to write out to a file prior to piping it into docker
* use $() instead of backticks
* tidied up messaging from failed docker commits