docs: add a note to the migration guide regarding using a Procfile for Dockerfile deploys
docs: add a note to the migration guide regarding using a Procfile for Dockerfile deploys

Closes #4769
claytonbrown/dokku,dokku/dokku,dokku/dokku,claytonbrown/dokku,claytonbrown/dokku,claytonbrown/dokku,dokku/dokku,dokku/dokku,claytonbrown/dokku,dokku/dokku,claytonbrown/dokku,dokku/dokku,claytonbrown/dokku,claytonbrown/dokku,dokku/dokku,dokku/dokku