Add a Dockerfile and document usage in README

Dockerfile.gpu

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+FROM tensorflow/tensorflow:1.12.0-gpu-py3
+
+# nvidia-docker 1.0
+LABEL com.nvidia.volumes.needed="nvidia_driver"
+LABEL com.nvidia.cuda.version="${CUDA_VERSION}"
+
+# nvidia-container-runtime
+ENV NVIDIA_VISIBLE_DEVICES=all \
+    NVIDIA_DRIVER_CAPABILITIES=compute,utility \
+    NVIDIA_REQUIRE_CUDA="cuda>=8.0" \
+    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