simplify the dockerfile

Dockerfile

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+# Dockerfile for running combine - https://github.com/mlsecproject/combine
+# written by Jose Nazario from Kyle Maxwell's
+
+MAINTAINER jose nazario [email protected]
+FROM python:2
+RUN groupadd -r combine && \
+    useradd -r -g combine -d /home/combine -s /sbin/nologin -c "Combine user" combine
+
+WORKDIR /home/combine
+USER combine
+ENV HOME /home/combine
+ENV USER combine
+ADD . /home/combine
+COPY combine-example.cfg /home/combine/combine.cfg
+USER root
+RUN pip install -r requirements.txt && \
+    chown -R combine:combine /home/combine 
+USER combine
+CMD ["python", "combine.py", "-e"]