Starting in Rails 7.1, Docker files are added by default in new applications, but Docker has been popular for Rails development for many years before that. At FastRuby.io, we use the Dual-Boot technique when we work on upgrades, and using that approach when an application uses Docker requires some extra steps to keep a great development experience.

If you search online for using SSH keys with Docker, to access private Github repositories, you will get a lot of search results, but the solutions you’ll find are almost all out of date, insecure, or fragmentary (i.e. they provide a small snippet of information but not a complete solution). Given how popular both Docker and Github are, I found this quite surprising. We recently had to set up Docker with a Rails application that fetches gems from private repositories. We’re also using Docker Compose, which added to the challenge. This comment on the Docker project, which is from February 2021, unfortunately is still accurate:

There are several questions and answers out there about how to pull from a private repository (using the hosts ssh key & config). A lot of them are not working, not secure or unanswered

After several hours of research and testing, we have a good solution to share. But first let’s take a look at the different approaches to consider.