WIP #189 Ensure that the Docker image has node_modules/

Signed-off-by: Remy Suen <remy.suen@gmail.com>

Dockerfile

@@ -1,5 +1,8 @@
 FROM node:alpine
 COPY lib /docker-langserver/lib
 COPY bin /docker-langserver/bin
-RUN chmod +x /docker-langserver/bin/docker-langserver
+COPY package.json /docker-langserver/package.json
+WORKDIR /docker-langserver/
+RUN npm install --production && \
+    chmod +x /docker-langserver/bin/docker-langserver
 ENTRYPOINT [ "/docker-langserver/bin/docker-langserver" ]