Switch the Dockerfile to use golang:1.3.3-cross as the base

This switches us to Debian Jessie, which as of Nov 5th is in [freeze](https://release.debian.org/jessie/freeze_policy.html), so any changes to "Jessie" as our turtle at the bottom of the stack from here out will be reasonably minor and easy to manage.

This also includes some moving around of bits of the Dockerfile to improve caching and consistency.

This is essentially #5214, take two.