Use --only=production on npm install

Add --only=production flag to Dockerfile and the README entries
describing how to install nodes.  This will avoid pulling in a lot of
dependencies that are not needed for running master/slaves.

Dockerfile

@@ -2,7 +2,7 @@ FROM node:9.11.2
 RUN apt-get update && apt install git curl tar -y
 RUN mkdir factorioClusterio
 
-RUN git clone -b master https://github.com/Danielv123/factorioClusterio.git && cd factorioClusterio && npm install
+RUN git clone -b master https://github.com/Danielv123/factorioClusterio.git && cd factorioClusterio && npm install --only=production
 RUN cd factorioClusterio && curl -o factorio.tar.gz -L https://www.factorio.com/get-download/latest/headless/linux64 && tar -xf factorio.tar.gz
 
 WORKDIR factorioClusterio

README.md

@@ -95,7 +95,7 @@ Master and all slaves:
     cd factorioClusterio
     curl -o factorio.tar.gz -L https://www.factorio.com/get-download/latest/headless/linux64
     tar -xf factorio.tar.gz
-    npm install
+    npm install --only=production
     cp config.json.dist config.json
     node ./lib/npmPostinstall.js
 
@@ -283,7 +283,7 @@ node client.js list
 ```
 git pull
 
-npm install
+npm install --only=production
 ```
 
 3. Download the latest version of the factorioClusterioMod from its github repo