Feature/dockerfile

Created by: emanueleparisi

Add features to the Dockerfile provided to make the resulting image more suitable for CVA6 software development. New Dockerfile features:

• Move base image from ubuntu:18.04 to ubuntu:20.04.
• Install libncurses-dev to allow Buildroot and Linux Kernel configuration from within the container.
• Add a non-root user inside the container.
• Download and build the RISC-V toolchain on docker image construction.
• Update the README.md accordingly with the performed changes.