Make PUBLIC_URL and REACT_APP_SERVER configurable (#266)

* Adjust Dockerfile to include args in build process
* Adjust README.md

Dockerfile

@@ -1,11 +1,14 @@
 # Builder
 FROM node:lts as builder
 
+ARG PUBLIC_URL=/
+ARG REACT_APP_SERVER
+
 WORKDIR /src
 
 COPY . /src
 RUN yarn --network-timeout=100000 install
-RUN yarn build
+RUN PUBLIC_URL=$PUBLIC_URL REACT_APP_SERVER=$REACT_APP_SERVER yarn build
 
 
 # App