Make can't find bapbundle and docker build relies on old ubuntu 18.04.

Build instructions for the repo are problematic:

* `docker build -t bap-toolkit .` fails because it can't find `docker.io/library/ubuntu:18.04` which is allegedly a rather old version.
* `make` fails because it can't find `bapbundle`.

For someone just trying to get out to play with the code, these build instructions are somewhat inadequate.