Add Dockerfile.

Build: docker build -t git_stats:latest .
Usage: docker run /path/to/repo:/src git_stats:latest

Dockerfile

@@ -0,0 +1,8 @@
+FROM debian:stretch-slim
+#RUN sed -i 's/deb.debian.org/mirrors.ustc.edu.cn/g' /etc/apt/sources.list
+RUN apt-get update \
+        && apt-get install -y git ruby ruby-nokogiri ruby-nokogiri-diff ruby-nokogumbo \
+        && gem install git_stats \
+        && rm -rf /var/lib/apt/lists/*
+WORKDIR /src
+CMD git_stats generate