[skip ci] minor updates on build_flowmq.sh

Author: Commelina

scripts/build_flowmq.sh

@@ -6,5 +6,5 @@ docker compose --file ./docker/docker-compose-flowmq.yml \
                --build-arg USE_CHINA_MIRROR=false \
                --build-arg arg_http_proxy="" \
                --build-arg arg_https_proxy="" \
-               --build-arg FLOWMQ_IMAGE=ghcr.io/flowmq-io/flowmq:main \
-               --build-arg BASE_IMAGE=jepsen-flowmq:base
+               --build-arg FLOWMQ_IMAGE=${FLOWMQ_IMAGE:-ghcr.io/flowmq-io/flowmq:main} \
+               --build-arg BASE_IMAGE=${BASE_IMAGE:-jepsen-flowmq:base}