Setting up dev containers in Rails

Update: Rails 7.2 is released with Rails devcontainer support. You can now run:

rails new myapp --devcontainer

If you want to add devcontainer to an existing rails app. You can run:

rails devcontainer

Which creates the devcontainer files for you:

output of running rails devcontainer. It figures out what libraries you use and creates the devcontainer folder and files for you.

Development containers, aka dev containers, have been around since 2019. Made by Microsoft, they allow you to use a container as a development environment. If you’ve used GitHub Codespaces, you’ve loaded a dev container. In Rails 7.2, there will be a new devcontainer flag that will generate a dev container folder for you. Ever since I wrote about Rails dev containers, I’ve been eager to try them. If you also want to be an early adopter, here are a few issues I ran into.

Installing libraries

Before I can get dev containers working locally, I need to install a few libraries and extensions.

VSCode dev container extension, which will run my application as a dev container inside VSCode.

Colima, which is my container runtime. Docker Desktop is another option. Due to licensing I can’t use Docker Desktop at work, which is why I use Colima.

brew install colima

Docker to run my containers.

brew install docker

Docker compose, which is needed to run the dev container compose.yaml file.

brew install docker-compose

I also updated my ~/.docker/config.json to add the plugin

"cliPluginsExtraDirs": [
	"/opt/homebrew/lib/docker/cli-plugins"
]

Setting up a Rails dev container

First off, I created a new Rails API app with the devcontainers flag:

rails new dev-containers-app --main --database=mysql --devcontainer

After opening the project in VSCode, you should get a popup asking if you want to Reopen in container. If you don’t, you can use command palette (cmd+shift+p for mac) and type dev container: Reopen in Container.

Issues with dev container

Update: It looks like with the latest Rails release, these issues have been fixed.

When I first ran the dev container, the bin/setup step errored:

ActiveRecord::ConnectionNotEstablished: Can't connect to local server through socket '/tmp/mysql.sock'

The issue is that config/database.yaml is trying to read a mysql.sock file that doesn’t exist in the container. The easiest fix is to connect to the DB through the network. Modify your config/database.yaml file with:

default: &default
  adapter: mysql2
  encoding: utf8mb4
  pool: <%= ENV.fetch("RAILS_MAX_THREADS") { 5 } %>
  username: root
  password:
  host: mysql
  port: 3306

After this change, I was able to get my dev container up and running. However, when I tried to do Git things like git status, I got an error:

fatal: detected dubious ownership in repository at '/workspaces/dev-containers-app'
To add an exception for this directory, call:
    git config --global --add safe.directory /workspaces/dev-containers-app

To resolve this issue, I updated the devcontainer.json file. Adding a postStartCommand.

{
	... // Rest of config
	"postStartCommand": "git config --global --add safe.directory ${containerWorkspaceFolder}"
}

Initial thoughts

Here’s what I loved:

Modifying files in the container also modified the files on my local machine. And vice versa! It’s a fast development workflow
Git just works! It pulled my gitconfig from my machine, and I could push verified commits from the container.
Since the app runs in a container, I don’t need to worry about dependency management any more. If I need a new dependency, I can pull it into my docker image.
It works out of the box with VSCode dev container extension.

Here’s what I didn’t like:

You can’t run --skip-bundle and generate a .devcontainer folder. Maybe someone can correct me if I’m wrong. I would like to not have to install anything locally if possible!
The best dev experience is with VSCode. If you don’t use that editor, your workflow would be more involved. You would want to use the devcontainers/cli.
If you have an underpowered machine, dev containers might not be for you. If your container run time isn’t beefy enough with RAM & CPU a-plenty, then your dev container might be sluggish.

GitHub repo for reference.