Add a Dockerfile and document usage in README

Dockerfile.cpu

@@ -0,0 +1,12 @@
+FROM tensorflow/tensorflow:1.12.0-py3
+
+ENV LANG=C.UTF-8
+RUN mkdir /gpt-2 
+WORKDIR /gpt-2
+COPY requirements.txt download_model.sh /gpt-2/
+RUN apt-get update && \
+    apt-get install -y curl && \
+    sh download_model.sh 117M
+RUN pip3 install -r requirements.txt
+
+ADD . /gpt-2