add CONTAINER_APP_ROOT ARG in docker file so it can be changed in run time

Dockerfile

@@ -1,11 +1,12 @@
 # build stage
 FROM node:lts-alpine as build-stage
+ARG CONTAINER_APP_ROOT="/app"
 
 RUN npm update npm -g
 RUN npm config set "@fortawesome:registry" https://npm.fontawesome.com/
 RUN npm config set "//npm.fontawesome.com/:_authToken" 58624E90-2685-43C6-BF0F-0BFECCE11CD2
 RUN npm install -g http-server
-WORKDIR /app
+WORKDIR ${CONTAINER_APP_ROOT}
 
 COPY frontend/package*.json ./