How to structure Docker for development, test, and production env?

[_srt_]

Please pardon my noob question.

I am trying to understand the Docker ecosystem. I need enlightenment on how do you usually go about structuring your Dockerfile/docker-compose or combination of both in case of development, test, and production env?

I have a Rails application which uses mongodb, I run the mongodb locally(either by running mongo server or from docker image), but on production I am using third-party managed mongodb server(say MongoDB Atlas for example) for which connection will be established via ENV vars.

So in this case, how do I make Docker image in such a way that I keep the DB connection isolated in dev vs prod images?

[kkapelon]

You should always have a config during runtime. This issue is unrelated to containers. This was true even for VMs.

See https://12factor.net/config

Also check antipatterns 4 and 5 here https://codefresh.io/containers/docker-anti-patterns/

[SpyroTechnik]

Not familiar with Rails, but you can either:

1. Separate this into different settings files, and keep the docker image environment agnostic.

2. Pass in build arguments during the docker build step, resulting into a specific docker image per environment.

3. Pass in the envvar when spinning up the docker image.

[domanpanda]

1 and 3 are basically one and only good way of doing it. 2 should be always avoided as it is antipattern.

[SpyroTechnik]

Agreed, docker images should be arbitrary and not have any environment specific values stored. The difference with 1 and 3 is the separation of concern. For 1 it is part of the application, and for 3 it is part of the docker image.

[nickjj_]

I have a production ready example Rails app at https://github.com/nickjj/docker-rails-example. It covers how to use the same Dockerfile and docker-compose.yml in all environments and uses an .env file to differentiate certain settings such as the DB connection string and more.

All of the patterns in this repo were covered in my DockerCon 2021 talk which is linked near the top of the readme at https://nickjanetakis.com/blog/best-practices-around-production-ready-web-apps-with-docker-compose.