Question: If I were to deploy a rails app using docker, can I put the whole stack in a single container (ruby/postgres/redis/sidekiq/etc.) Or do I need a separate container for each process?
Also, while you're likely to find pre-packaged dockerfiles for each of your components, the odds are lower that you'll find an existing dockerfile that has your exact stack all-in-one. So, while the "one container" approach may save you some cross-container configuration overhead, it will likely cost you extra dockerfile creation time.
In fact, I think you should run multiple processes because running just a single process is wrong. It will not enable many kinds of important system services, such as syslog. Take a look at https://github.com/phusion/baseimage-docker for a full explanation.
This is also the reason why I wrote baseimage-docker: to make it easy for other Docker users to get things right. I highly encourage people to use baseimage-docker.
> And having separate ENTRYPOINT and USER directives suggests that if you want to run several things at once, they must all run as the same user.
It does not. That's not how the Unix process model works.
When you boot the Linux kernel, it also wants you to specify a single entry point. The kernel starts that process as root. Yet your Linux system clearly has multiple processes, running as different users. Why? It's because the entry point is the init system, which spawns other processes. You can do the same thing inside Docker.
It's exactly the same as how your host system works! Nothing has changed!
I look at Docker as a lightweight way of modeling infrastructure and services. I can built a shipload/stack of containers that model/mock/replicate my application architecture or my SOA model or my three tier web application.
At the crudest level I can do this by modeling each host (web/app/db for example) with a container (hence single service) but it has the potential to scale up to large, clustered SOA applications too. Having that local model of my application means I am developing something that is a lot closer to reality than running a bunch of services on a shrunken set/consolidated set of hosts (or worse on my desktop) in a dev/test environment.
Apologies for confusing the situation.