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

README.md

@@ -66,6 +66,9 @@ or by editing it in the [.env](.env) file. See also the
         context: https://github.com/Awesome-Technologies/synapse-admin.git
         # args:
         #   - NODE_OPTIONS="--max_old_space_size=1024"
+        #   # see #266
+        #   - PUBLIC_URL="/synapse-admin"
+        #   - REACT_APP_SERVER="https://matrix.example.com"
       ports:
         - "8080:80"
       restart: unless-stopped