Integrate Docker into the build

- Isolates us from the host -- no more accidentally using a feature
that's present on a newer distro but not the build machine; no more
relying on packages you didn't realize you installed
- Both continuous build and rootfs leverage this, as well as local
builds

Continuous and rootfs build results on Kokoro w/ this change:
- https://sponge.corp.google.com/target?id=c061e5b2-b94f-4056-bf25-c69e25a71ed3&target=spacepark/enterprise/continuous-test
- https://sponge.corp.google.com/target?id=d6f312f2-9610-4be1-b7f1-cc7e7c7db6d9&target=spacepark/enterprise/rootfs-test

Change-Id: I95b305054ed16269b83bbcbaf89d8b89d8628fad
10 files changed