This is a bug with the baserow/baserow:1.14.0 and below images and has been fixed in baserow/baserow:develop-latest, see this thread. Base docker image needs "procps" apt package, otherwise "pgrep" is missing
It will be in baserow/baserow:1.15.0 and onwards once released, for now you can switch your docker images to build from baserow/baserow:develop-latest.
But once again the warning you are seeing is just a warning, the makemigrations command
docker-compose -f docker-compose.dev.yml run --rm my-baserow-plugin backend-cmd manage makemigrations
I believe is actually succeeding for you