Use python:3-slim as a base image

Dockerfile

@@ -9,7 +9,7 @@ RUN apk add --no-cache curl && \
     install -b -c -v /tmp/tflint /usr/local/bin/ && \
     rm -rf /tmp/*
 
-FROM python:3
+FROM python:3-slim
 COPY requirements.txt /requirements.txt
 RUN pip --no-cache-dir install -r requirements.txt
 ADD https://raw.githubusercontent.com/saltstack/salt/master/.pylintrc /etc/saltpylint