Add dockerfile

.dockerignore

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+node_modules/
+Dockerfile

Dockerfile

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+FROM alpine:3.5
+MAINTAINER GoCD Contributors <go-cd-dev@googlegroups.com>
+
+EXPOSE 5000
+
+COPY . /cla-assistant
+WORKDIR /cla-assistant
+
+RUN \
+  apk add --update nodejs su-exec git curl bzip2 patch make g++ && \
+  addgroup -S cla-assistant && \
+  adduser -S -D -G cla-assistant cla-assistant && \
+  chown -R cla-assistant:cla-assistant /cla-assistant && \
+  su-exec cla-assistant /bin/sh -c 'cd /cla-assistant && npm install && node_modules/grunt-cli/bin/grunt build && rm -rf /home/cla-assistant/.npm .git' && \
+  apk del git curl bzip2 patch make g++
+
+USER cla-assistant
+CMD npm start