fix python dependencies build error and shrink docker image size

[kmahyyg]

The philosophy of Docker is simple, small and easy.

Reason why I did this:

• Jessie is an old debian version, we should upgrade to latest.
• apt relies on python, python on jessie is 3.4, which means you have both 3.4 and 3.6 in your docker image, it's dirty.
• python in docker-compose.yml should switch to python3, since the default python of the most old distro is python2.
• According to the philosophy of docker, I upgraded the debian to latest and only use the latest (3.8 here) python in a single image, which avoided two python works in a image.
• Docker is running using overlay storage structure, so you should combine the shell command which runs for the same purpose in a single RUN row to shrink the image size.
• Due to the upgrade of Python and distro, I just fixed build error when running pip3 install -r, so I changed the requirements.txt and built myself.

The image built on my machine is successfully finished and runs perfectly. Works fine for me.

[kmahyyg]

REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
mqtt-pwn_cli        latest              a8120b7894cc        11 minutes ago      273MB

This also fixed Issue #7