Use NodeBB 1.7.5

Dockerfile

@@ -7,7 +7,7 @@ WORKDIR /app/code
 ENV PATH /usr/local/node-6.9.5/bin:$PATH
 
 # This uses 1.7.4 + the fix for https://github.com/NodeBB/NodeBB/pull/6304
-RUN curl -L https://github.com/NodeBB/NodeBB/archive/8c9bae8ba388f94750130819106d240e90715be7.tar.gz | tar -xz --strip-components 1 -f -
+RUN curl -L https://github.com/NodeBB/NodeBB/archive/1.7.5.tar.gz | tar -xz --strip-components 1 -f -
 ADD config.json.template /app/code
 RUN cp /app/code/install/package.json /app/code/package.json
 RUN npm install --production