You mention setting up a Phoenix app in Docker so I'll link to a couple of posts I wrote describing how I do that: https://dev.bleacherreport.com/starting-a-phoenix-project-wi..., https://dev.bleacherreport.com/brunch-in-a-container-phoenix.... I almost always start new Elixir projects in Docker for local development now, and deploy the images to Elastic Beanstalk. If you're not setting up a cluster it's not a bad option.

At Bleacher Report, we've definitely reduced operating costs on some of our services by switching to Elixir and running on fewer/smaller machines, and we routinely pick Elixir for new services now. But we also recently launched a Node service which could handle all frontend traffic on a single server, so in some ways it's hard to say whether the gains are more due to the technology or improvements in the service design.