This updates all references to SDL 2.0.22 to SDL 2.24.0, including the
Docker container that I maintain for Linux CI.
Be warned, this release of SDL updates the versioning scheme to be less
dumb. The previous version is 2.0.22, this release is 2.24.0 (so the
last number can be properly used for patch-level version releases).
2.0.22 just released 40 minutes ago.
This also updates the `Dockerfile` to use the URL from the GitHub
releases page, instead of SDL's servers.
I've also pushed a new Docker container to
`ghcr.io/infoteddy/vvvvvv-build`.
