Update outdated README.md of DUNE

[pliuwu1998]

Some important info as below should be added into README.md:

1. Describe how to clone and debug DUNE locally using ./dune (not just globally installed dune)
2. indicate how to use interactive bash of docker to let end users able to use dune_container as a local computer such as check nodeos output, ps aux to find running nodeos, ls nodes/ etc.
3. Indicate use of 'pytest -v tests/' to do unit test, and before that use ./bootstrap.sh