Recently it comes to my attention that Github Actions image installs
experimental C++ standard library from an unsupported PPA, which
effectively breaks assumptions of binary compatibility for the OS
(in this case, Ubuntu 18.04+).
Unfortunately, as a result, the Github Actions environment can no
longer be trusted to compile proper binaries.
This change adds a Dockerfile that composes a proper Ubuntu 18.04
environment from official "ubuntu:bionic" image, and builds Node
binaries in it.
Bug: actions/virtual-environments#3432
