Force pip version to 20.1 to avoid build issues

See pypa/pip#7309. Using --install-option and
either of --prefix or --install-lib results in an error starting at pip
20.2.

Author: pingtimeout

Dockerfile

@@ -24,7 +24,7 @@ RUN apt-get update && apt-get install -y \
     unzip \
     wget
 
-RUN pip install --upgrade pip
+RUN pip install --upgrade pip==20.1
 RUN pip install \
     six \
     urllib3 \