Add comments to dockerfile

Dockerfile

 FROM node:10.15.3-alpine
 
+# Create app directory
 RUN mkdir /opt/app && chown node:node /opt/app
 
 RUN apk add --update git && \
 rm -rf /tmp/* /var/cache/apk/*
 
-# Create app directory
 WORKDIR /opt/app
 
 USER node
@@ -22,8 +22,10 @@ COPY .babelrc ./
 # Bundle app source
 COPY src ./src
 
+# Run the scripts defined in package.json
 RUN npm install && npm run build
 
 EXPOSE 3001
 
+# Express server handles the backend functionality and also serves the React app
 CMD ["node", "dist/server"]