title: container builds and whitelisting of dpkg
body: <p>It would be nice if there was a "sudo" option which meant: I've made my repo container happy, but I need packages not yet whitelisted. (which is why I'm using sudo)</p>
  
  <p>Then travis-ci would know which packages to prioritize getting whitelisted: this would accelerate the process of getting more source trees out of the legacy (and presumably more costly) VMs and into containers.</p>
  
  <p>It would best if addon/package list, if it couldn't be satisfied from container safe packages, automatically became a legacy build, and then used sudo apt-get install to install the rest.  </p>
  
  <p>That would require analysis of the addon list prior to committing to legacy/container build, but would also mean that as packages are whitelisted, 
  packages would move from the legacy to container build automatically.</p>