Docker is a nice way of doing build automation, or just
containerizing builds for increased safety and isolating unstable
packages. The old Dockerfile in the toolchain did not satisfy these
needs. The new Dockerfile is known to run successfully on Docker
version 20.10.7. It clones the SerenityOS repo and builds the
toolchain. In this way, it is intended to be a starting point for other
Docker images that can e.g. run builds. For example, one can simply run
this docker image as-is, exec a shell in it and run a build there.
