Add flake8 to Dockerfile

requirements.txt

@@ -7,6 +7,7 @@ pytest-cov>=2.7.1
 Sphinx>=2.1.2
 sphinxcontrib-napoleon>=0.7
 Pygments>=2.4.2
+flake8>=3.8.3
 
 # IDE related (can be skipped)
 pylint>=2.3.1