Move all Linux CI jobs to Docker

[elliottslaughter]

Having given up on #599, I am moving all Linux tests to Docker. This is the only sane way to be isolated from periodic changes in the GitHub runners.

Issues to be decided and/or implemented:

• [x] Makefile on Linux
• [x] no-static / no-slib
• [x] luajit/moonjit
• [x] Review coverage and make sure we're testing what we want/need

[elliottslaughter]

Confirmed the binaries on:

• Linux ARMv8 Ubuntu 18.04
• Linux x86_64 Ubuntu 20.04 CUDA 11.1
• Linux PPC64le RHEL 8