New Answer
Docker Compose has been updated. They now have a version 2 file format.
Version 2 files are supported by Compose 1.6.0+ and require a Docker Engine of version 1.10.0+.
They now support the networking feature of Docker which when run sets up a default network called myapp_default
From their documentation your file would look something like the below:
version: '2'
services:
web:
build: .
ports:
- "8000:8000"
fpm:
image: phpfpm
nginx
image: nginx
As these containers are automatically added to the default myapp_default network they would be able to talk to each other. You would then have in the Nginx config:
fastcgi_pass fpm:9000;
Also as mentioned by @treeface in the comments remember to ensure PHP-FPM is listening on port 9000, this can be done by editing /etc/php5/fpm/pool.d/www.conf where you will need listen = 9000.
Old Answer
I have kept the below here for those using older version of Docker/Docker compose and would like the information.
I kept stumbling upon this question on google when trying to find an answer to this question but it was not quite what I was looking for due to the Q/A emphasis on docker-compose (which at the time of writing only has experimental support for docker networking features). So here is my take on what I have learnt.
Docker has recently deprecated its link feature in favour of its networks feature
Therefore using the Docker Networks feature you can link containers by following these steps. For full explanations on options read up on the docs linked previously.
First create your network
docker network create --driver bridge mynetwork
Next run your PHP-FPM container ensuring you open up port 9000 and assign to your new network (mynetwork).
docker run -d -p 9000 --net mynetwork --name php-fpm php:fpm
The important bit here is the --name php-fpm at the end of the command which is the name, we will need this later.
Next run your Nginx container again assign to the network you created.
docker run --net mynetwork --name nginx -d -p 80:80 nginx:latest
For the PHP and Nginx containers you can also add in --volumes-from commands etc as required.
Now comes the Nginx configuration. Which should look something similar to this:
server {
listen 80;
server_name localhost;
root /path/to/my/webroot;
index index.html index.htm index.php;
location / {
try_files $uri $uri/ /index.php?$query_string;
}
location ~ \.php$ {
fastcgi_split_path_info ^(.+\.php)(/.+)$;
fastcgi_pass php-fpm:9000;
fastcgi_index index.php;
include fastcgi_params;
}
}
Notice the fastcgi_pass php-fpm:9000; in the location block. Thats saying contact container php-fpm on port 9000. When you add containers to a Docker bridge network they all automatically get a hosts file update which puts in their container name against their IP address. So when Nginx sees that it will know to contact the PHP-FPM container you named php-fpm earlier and assigned to your mynetwork Docker network.
You can add that Nginx config either during the build process of your Docker container or afterwards its up to you.