Don't start systemctl service, like Docker, unless a control file is present
You can add a path unit to monitor a filesystem path and start the corresponding service. You can create a docker.path
unit to activate a corresponding docker.service
unit. It might look something like this:
[Unit]
Description=Start Docker when my file exists
[Path]
PathExists=/my/path
[Install]
WantedBy=multi-user.target