We want it to complain loudly if a dependency is missing, not just
quietly continue on.

Might be useful for testing in some cases. Note that the Windows and
Linux binaries will depend on libraries from the build environment. So
the user will need to match those manually.

CentOS 8 is EOL and is now replaced with CentOS Stream 8.

This images isn't available on Docker Hub unfortunately, so we grab it
from Quay.

It's difficult to support both old and new versions, so raise the
requirement to the oldest that is commonly used, which is CMake 3.10.2
on Ubuntu 18.04.

This version of Ubuntu is no longer fully supported so let's stop caring
about builds there.

Travis has unfortunately been severely rate limited by Docker Hub so we
have to switch to GitHub, which has an agreement with Docker to avoid
the rate limiting.