Fix CI: Use 18.04 base docker image + run lint w/ py3.8 (#532)

### Summary:

Fix CI builds by upgrading the base docker image to 18.04 and running lint with py3.8.

The ppa we use to install python in our docker images recently dropped support for Ubuntu 16.04.

Also, Python 3.6 recently passed EOL so pip complains and fails during the lint build

### Test Plan:

CI

build/ci/install_lint_deps.sh

@@ -2,12 +2,12 @@
 set -e
 
 # We need a newer version of libstdc++ for infer to run
-# We also need python3.6 for black to run
+# We also need python3.8 for black to run
 apt-get install -y --no-install-recommends software-properties-common
 add-apt-repository -y ppa:ubuntu-toolchain-r/test
 add-apt-repository -y ppa:deadsnakes/ppa
 apt-get update
-apt-get install -y --no-install-recommends libstdc++6 python3.6 python3.6-dev tzdata jq
+DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends libstdc++6 python3.8 python3.8-dev tzdata jq
 apt-get install -y git
 
 # Install infer
@@ -21,8 +21,8 @@ wget https://raw.githubusercontent.com/Sarcasm/run-clang-format/de6e8ca07d171a7f
 
 # Install pip, black, and flake8
 wget https://bootstrap.pypa.io/get-pip.py
-python3.6 get-pip.py
-python3.6 -m pip install black==19.10b0 flake8
+python3.8 get-pip.py
+python3.8 -m pip install black==19.10b0 flake8
 
 # Get buildifier (for linting bazel files)
 wget https://github.com/bazelbuild/buildtools/releases/download/2.2.1/buildifier -O /tmp/buildifier

build/install_python_deps.sh

@@ -5,7 +5,7 @@ set -e
 NEUROPOD_PYTHON_BINARY="python${NEUROPOD_PYTHON_VERSION}"
 
 # Install pip
-wget https://bootstrap.pypa.io/pip/2.7/get-pip.py -O /tmp/get-pip.py
+wget https://bootstrap.pypa.io/pip/3.6/get-pip.py -O /tmp/get-pip.py
 ${NEUROPOD_PYTHON_BINARY} /tmp/get-pip.py
 
 # Setup a virtualenv

build/neuropod.dockerfile

@@ -4,7 +4,7 @@
 # FROM ubuntu:16.04 as neuropod-base
 ARG NEUROPOD_CUDA_VERSION=10.0
 ARG NEUROPOD_CUDNN_VERSION=7
-FROM nvidia/cuda:${NEUROPOD_CUDA_VERSION}-cudnn${NEUROPOD_CUDNN_VERSION}-runtime-ubuntu16.04 as neuropod-base
+FROM nvidia/cuda:${NEUROPOD_CUDA_VERSION}-cudnn${NEUROPOD_CUDNN_VERSION}-runtime-ubuntu18.04 as neuropod-base
 
 # Use utf8
 ENV LC_ALL=C.UTF-8
@@ -34,15 +34,11 @@ ENV NEUROPOD_PYTHON_VERSION=$NEUROPOD_PYTHON_VERSION
 
 # Install python
 RUN sudo apt-get update && \
-    sudo apt-get install -y "python${NEUROPOD_PYTHON_VERSION}" "python${NEUROPOD_PYTHON_VERSION}-dev" && \
+    sudo apt-get install -y "python${NEUROPOD_PYTHON_VERSION}" \
+                            "python${NEUROPOD_PYTHON_VERSION}-dev" \
+                            "python${NEUROPOD_PYTHON_VERSION}-distutils" && \
     ln -s "$(which python3)" /usr/bin/python
 
-# For python 3.5, we need to install ffi
-RUN if [ "${NEUROPOD_PYTHON_VERSION}" = "3.5" ] ; then sudo apt-get install -y libffi6 libffi-dev ; fi
-
-# For python 3.8, we need to install distutils
-RUN if [ "${NEUROPOD_PYTHON_VERSION}" = "3.8" ] ; then sudo apt-get install -y "python${NEUROPOD_PYTHON_VERSION}-distutils" ; fi
-
 # Copy the python code into the image
 RUN mkdir -p /usr/src/source/python /usr/src/source/neuropod/python
 COPY build/install_python_deps.sh /usr/src/build/install_python_deps.sh