Add working dockerised frontend

sg-frontend/.env

-VITE_BACKEND_URL=
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+VITE_BACKEND_URL=http://localhost:8000
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sg-frontend/Dockerfile

@@ -5,7 +5,7 @@ COPY . .
 RUN npm install
 
 RUN npm run build
-
+ENV VITE_BACKEND_URL=http://sg-backend:8000
 EXPOSE 4173
 
 CMD ["npm","run", "preview", "--","--host", "0.0.0.0", "--port", "4173"]
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