Switch to python 3.6-slim image, add lxml and requests

Dockerfile

-FROM python:3-alpine
+FROM python:3.6-slim
 
-RUN apk update && \
-    apk add bash git shellcheck curl && \
-    git config --global credential.helper store && \
-    pip install -U flake8 pylint
+RUN export DEBIAN_FRONTEND=noninteractive && \
+    apt-get update && \
+    apt-get -y install bash git shellcheck curl libxml2 libxslt1.1 && \
+    rm -rf /var/lib/apt/lists/*
+
+RUN pip install -U flake8 pylint lxml ast-to-xml requests