What is the right way to run multiple processes from my start command?

Using a worker or multiple application containers is the recommended way of starting more than one process. But if you are not able to use a worker and have no other way, it is technically possible to use supervisord to start multiple processes. Please only use this as a last resort.

Example of running supervisord to run both PHP and dropboxd

dependencies:
  python:
    supervisor: '*'

web:
    commands:
        start: "supervisord -n -c supervisor.conf"

supervisor.conf gets committed to the repository and looks like this:

[supervisord]
logfile=/tmp/supervisord.log
nodaemon=true
minfds=1024
pidfile=/tmp/supervisord.pid

[program:php]
command=/usr/bin/start-php-app
process_name=%(program_name)s
autostart=true
autorestart=true

[program:dropbox]
process_name=%(program_name)s
command=/app/.dropbox-dist/dropboxd
autostart=true
autorestart=true