Modern applications have a lot of moving parts. A web application almost always uses a database such as MySQL or MongoDB to store data. There may be a load balancer or SSL proxy. Often there are tasks scheduled to run regularly that perform routine maintenance or backups. On a traditional server, these tasks might all be installed on a single server or spread across numerous hosts. The challenge faced by system administrators is how are all these parts deployed consistently and reliably?
Rather than having a single container that does everything, you can split your application into smaller chunks. Each container has a specific task and does only that task. For example, you might have one image that is a Perl-based web application; a second that runs MySQL; a third that runs HAProxy to load balance and provide SSL termination for the Perl application. Each separate container can be run, upgraded, and tested independent of...