Docker: issuing S2/host docker commands

Hello,

I think I have this working in a docker container, thank you.
One caveat though is that I am manually stopping the plex docker container on S2. I probably misunderstand something about the intent, wondering if anyone can tell me what I'm missing?

Given:
S1 is a docker container whose host is accessible by SSH
S2 is a docker container running on the local docker instance
plex-db-sync is a guest docker container running on the same local host as S2

I don't understand how the guest plex-db-sync docker container can run these docker commands against the host:

- S2_START=cd /docker; docker-compose up -d plex
- S2_STOP=cd /docker; docker-compose stop plex