support/dockerfile: install flake8

support/dockerfile: install flake8

Use the latest version of the tool because it is actively maintained.
But use a fixed version of the tool and its dependencies to get stable
results. It can be manually bumped from time to time.

Before installing any Python packages, ensure pip, setuptools, and wheel
are up to date as recommended in the docs [1].

[1] https://packaging.python.org/tutorials/installing-packages/

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Peter Korsgaard <peter@korsgaard.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Yann E. MORIN <yann.morin.1998@free.fr>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>