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>

support/docker/Dockerfile

@@ -27,10 +27,19 @@ RUN dpkg --add-architecture i386 && \
         cvs bzr git mercurial subversion wget \
         cpio unzip \
         libncurses5-dev \
-        python-nose2 python-pexpect qemu-system-arm qemu-system-x86 && \
+        python-nose2 python-pexpect qemu-system-arm qemu-system-x86 \
+        python-pip && \
     apt-get -y autoremove && \
     apt-get -y clean
 
+# For check-flake8
+RUN python -m pip install --upgrade pip setuptools wheel && \
+    pip install -q \
+        flake8==3.5.0 \
+        mccabe==0.6.1 \
+        pycodestyle==2.3.1 \
+        pyflakes==1.6.0
+
 # To be able to generate a toolchain with locales, enable one UTF-8 locale
 RUN sed -i 's/# \(en_US.UTF-8\)/\1/' /etc/locale.gen && \
     /usr/sbin/locale-gen