Why starting docker (mac desktop version) will start redis server as well

General
1

I found that start docker will start redis server as well. But I can’t find document for that behaviors, so why is that ? I tried latest docker desktop 2.0.0.2 macOS 10.14

qiulangs-MacBook-Pro:~ qiulang$ redis-cli ping
PONG
qiulangs-MacBook-Pro:~ qiulang$ docker --version
Docker version 18.09.1, build 4c52b90
qiulangs-MacBook-Pro:~ qiulang$ docker-compose --version
docker-compose version 1.23.2, build 1110ad01
qiulangs-MacBook-Pro:~ qiulang$ docker-machine --version
docker-machine version 0.16.1, build cce350d7

Thanks.