Update src/config/app.js, Dockerfile, .gitlab-ci.yml

Dockerfile

@@ -3,8 +3,8 @@ WORKDIR /usr/src/app
 COPY package.json yarn.lock ./
 RUN yarn
 COPY . ./
-ARG NEXT_PUBLIC_BASE_URL
-ENV REACT_APP_API_BASE_URL $NEXT_PUBLIC_BASE_URL
+ARG REACT_APP_BASE_URL
+ENV REACT_APP_BASE_URL $REACT_APP_BASE_URL
 RUN yarn build
 
 FROM nginx:stable-alpine