[-] Upgrade Python for test Docker image to 3.6

Dockerfile

@@ -2,16 +2,31 @@ FROM ubuntu:xenial
 
 ENV BLENDER_VERSION 2.81
 
+# Add Python 3.6 package repo
 RUN apt-get update
+RUN apt-get install -y software-properties-common
+RUN add-apt-repository ppa:deadsnakes/ppa
+
+# Update/upgrade and install system dependencies
+RUN apt-get update
+RUN apt-get upgrade -y
 RUN apt-get install --no-install-recommends -y \
     libsdl1.2debian \
-    libglu1 python3-pip \
+    libglu1 \
+    python3.6 \
+    python3.6-dev \
+    python3.6-venv \
     bash \
     wget \
     bzip2 \
     make \
     libxi6 \
     libxrender1
+
+# Retrieve and install pip for version 3.6 (not in above PPA)
+RUN wget https://bootstrap.pypa.io/get-pip.py
+RUN python3.6 get-pip.py
+
 RUN pip3 install --upgrade pip
 RUN pip3 install --upgrade setuptools