Rework configuration process

Dynamically loads `config.json` on startup.

Fixes #167, #284, #449, #486

Change-Id: I9efb1079c0c88e6e0272c5fda734a367aa8f84a3

Dockerfile

@@ -1,13 +1,12 @@
 # Builder
 FROM node:lts as builder
 LABEL org.opencontainers.image.url=https://github.com/Awesome-Technologies/synapse-admin org.opencontainers.image.source=https://github.com/Awesome-Technologies/synapse-admin
-ARG REACT_APP_SERVER
 
 WORKDIR /src
 
 COPY . /src
 RUN yarn --network-timeout=300000 install --immutable
-RUN REACT_APP_SERVER=$REACT_APP_SERVER yarn build
+RUN yarn build
 
 
 # App