Update docker section to use docker linking instead of docker ip.

README.md

@@ -26,11 +26,11 @@ your organization.
 ```bash
 $ docker run -d -p 5432:5432 -e POSTGRES_PASSWORD=mypass --name=postgres postgres
 $ docker run -d -p 6379:6379 --name=redis redis
-$ docker run --rm -e DATABASE_URL="postgresql://postgres:mypass@172.17.42.1/asciinema" asciinema/asciinema.org bundle exec rake db:setup
+$ docker run --rm --link postgres:postgres -e DATABASE_URL="postgresql://postgres:mypass@postgres/asciinema" asciinema/asciinema.org bundle exec rake db:setup
 # starting sidekiq using the provided start_sidekiq.rb file will also start sendmail service if you don't want to use SMTP
 # otherwise start sidekiq by starting: bundle exec sidekiq
-$ docker run -d -e DATABASE_URL="postgresql://postgres:mypass@172.17.42.1/asciinema" asciinema/asciinema.org ruby  start_sidekiq.rb
-$ docker run -d -e DATABASE_URL="postgresql://postgres:mypass@172.17.42.1/asciinema" -p 3000:3000 asciinema/asciinema.org
+$ docker run -d --link postgres:postgres -e DATABASE_URL="postgresql://postgres:mypass@postgres/asciinema" asciinema/asciinema.org ruby  start_sidekiq.rb
+$ docker run -d --link postgres:postgres -e DATABASE_URL="postgresql://postgres:mypass@postgres/asciinema" -p 3000:3000 asciinema/asciinema.org
 ```
 
 You can override the address/port that is sent in email with login token by passing HOST="host:port" environment variable when starting the web server.