I have no clue what a Procfile is, but usually I’d start services via systemd and let it handle dependencies as well.
In development its of course a bit different, there I’d have a script which kicks off and shuts down a bunch of docker containers for external services.
Also as long as your other stuff is running on the BEAM itself, you can add them as dependency and they will be started by the BEAM during your Apps start up process.
If I actually got your question wrong, please refine 
