build: inject env vars before npm run build

Dockerfile

@@ -27,6 +27,8 @@ COPY . .
 # Uncomment the following line in case you want to disable telemetry during the build.
 # ENV NEXT_TELEMETRY_DISABLED 1
 
+ENV NEXT_PUBLIC_CHAT_API https://praneeth-hakeem-patrick-backend.hf.space/api/chat
+
 RUN \
     if [ -f yarn.lock ]; then yarn run build; \
     elif [ -f package-lock.json ]; then npm run build; \