Previously we were somewhat half-Docker, half-Travis, half apt-get, etc. This
commit alters the CI infrastructure to use Docker images for each target. This
should make it much easier to update the images and tweak various bits and
pieces of installed software. Additionally, it's also much clearer now what's
needed for each suite of tests!

Some images were updated a bit (e.g. musl is now 1.1.14), but other images
encountered failures when updating so they're not getting updated just yet.