ci: docker - add flake8, remove infer

We do not use Infer after all (see MR !435) so it does not make sense to
have it in the image.

ci/Dockerfile

@@ -54,6 +54,7 @@ RUN rm -rf /tmp/resolver-benchmarking
 
 # Python static analysis for respdiff
 RUN pip3 install mypy
+RUN pip3 install flake8
 
 # Unbound for respdiff
 RUN apt-get install unbound unbound-anchor -y -qqq
@@ -72,6 +73,3 @@ RUN luarocks install luacov
 
 # LuaJIT binary for stand-alone scripting
 RUN apt-get install -y -qqq luajit
-
-# infer for static analysis
-RUN wget -q -O - https://github.com/facebook/infer/releases/download/v0.13.0/infer-linux64-v0.13.0.tar.xz | tar xJ && mv infer-linux64* /var/opt/infer