Updated answer (Oct 2017)
As others mentioned, the Docker API has changed. I’m updating my answer since it’s the one most people will probably look at.
docker-compose up -d --scale app=5
Unfortunately, we cannot specify this in a docker-compose.yml file currently (as of version 3.5).
Details:
They did introduce the scale option for version 2.2 and 2.3 of docker-compose, but they removed it for version 3.0. Also, to use version 2.2 or 2.3 you would need to download an older version of the docker-compose tool. The current version does not support 2.2 or 2.3 (it does support 2.0 or 2.1 however).
There is also a new deploy section with replicas: 5 but it’s only for swarm mode.
— Old Answer ————————————–
docker-compose scale app=5
See docker compose up.
Then you only need this docker-compose file:
version: '2'
services:
app:
image: app