refactored Dockerfile to account for asyncio's dependencies and replaced Pypy 3.7 with Python 3.9.2 as base

[m0rcq]

• refactored Dockerfile to account for asyncio's dependencies
• using requirements.txt as for pip in container build
• switched from pypy 3.7 to python 3.9.2 as base
• amended README.md

@seb-at-nzrs: the new asyncio dependencies as async_lru were breaking the container run, the above refactoring addresses this and also replaces Pypy with CPython 3.9.2 as discussed.