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

[gl-yziquel]

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.