caddy not recognizing env var

I'm using caddy server in docker to reverse proxy to my nodejs servers. I'm passing UPSTREAMS env var as documented Here. But the UPSTREAMS are not recognized by caddy. reverse_proxy works fine if I hardcode the UPSTREAMS in Caddyfile.

Dockerfile

# https://caddyserver.com/docs/build#docker

FROM caddy:2.8.4-builder AS builder

RUN xcaddy build \
    --with github.com/mholt/caddy-ratelimit

FROM caddy:2.8.4

COPY --from=builder /usr/bin/caddy /usr/bin/caddy

docker-compose.yaml

services:
  # https://caddyserver.com/docs/running#setup
  caddy:
    build:
      context: .
      dockerfile: Dockerfile.caddy
    container_name: caddy
    restart: unless-stopped
    environment:
      - DOMAIN=${DOMAIN:-http://localhost:80}
      - CERT_EMAIL=$CERT_EMAIL
      - UPSTREAMS="node-server-1:5000 node-server-2:5000"
    volumes:
      - ./Caddyfile:/etc/caddy/Caddyfile
      - ./caddy_data:/data
      - ./caddy_config:/config
    ports:
      - 80:80
      - 443:443

And my Caddyfile

{
    email {$CERT_EMAIL}
    # https://github.com/mholt/caddy-ratelimit?tab=readme-ov-file#caddyfile-config
    order rate_limit before basicauth

    log {
        level ERROR
    }
}

{$DOMAIN} {
    rate_limit {
        zone myzone {
            key {remote_ip}
            events 20
            window 1m
        }
    }

    handle {
        reverse_proxy {
            # THIS WORKS
            # to node-server-1:5000 node-server-2:5000


            # THIS DOESN'T WORK
                to {$UPSTREAMS}


            # https://caddyserver.com/docs/caddyfile/directives/reverse_proxy#load-balancing
            lb_policy round_robin

            lb_try_duration 1s
        }
    }

    # https://caddyserver.com/docs/caddyfile/directives/handle_errors#examples
    handle_errors 429 {
        respond "You're being rate limited. Please try again in 1 minute."
    }
}

Removing the quotes from UPSTREAMS env var solved the problem

    environment:
      - UPSTREAMS=node-server-1:5000 node-server-2:5000