Download binary shellcheck from upstream

Dockerfile

@@ -2,7 +2,14 @@ FROM python:3.6-slim
 
 RUN export DEBIAN_FRONTEND=noninteractive && \
     apt-get update && \
-    apt-get -y install bash git shellcheck curl libxml2 libxslt1.1 && \
+    apt-get -y install bash git curl libxml2 libxslt1.1 xz-utils && \
     rm -rf /var/lib/apt/lists/*
 
 RUN pip install -U flake8 pylint lxml ast-to-xml requests
+
+RUN mkdir -p /opt/shellcheck && \
+    cd /opt/shellcheck && \
+    curl -LO https://github.com/koalaman/shellcheck/releases/download/v0.7.2/shellcheck-v0.7.2.linux.x86_64.tar.xz && \
+    tar -xf *.tar.xz --strip-components=1 && \
+    rm *.tar.xz && \
+    ln -s /opt/shellcheck/shellcheck /usr/bin