[Dev] Improve onboarding experience for contributors

[kieraneglin]

Problem Statement

I'm addressing three main issues:

• Environment setup is often a large bottleneck for new contributors. I've lost count of the times where setting up a new project took 5x longer than writing the code change I wanted to contribute
• I have a lot of Ruby projects on the go and even with a version manager it's often a pain to keep my dev environment free of cross-contamination. Containerizing your development environment keeps your projects independent from changes on your host machine
• Many people (myself included) like having only the necessary extensions enabled in my IDE. I don't want a ton of Python/Rust/Elixir add-ons cluttering up a Ruby project

What it does

This PR adds a complete dev container setup to make getting started with inertia-rails development super straightforward. The project can be fully set up and ready for contributions just by opening a code editor that supports devcontainers!

Even if you don't use an editor that supports devcontainers, the provided Docker files also allow savvy users to get up-and-running quickly.

I'm told that this will also allow contribution via GitHub Codespaces but I haven't tested that

Notes

I don't mind at all if you don't accept this PR! It's not high effort but it helped me get set up while I look at #247.

If you're interested in this, I can add a little blurb on this in the README if you'd like 🤙

[kieraneglin]

This is needed since the container specifies as RAILS_ENV that would override this. You could instead use RAILS_ENV=test bundle exec rspec, but I can't think of a scenario in which you'd want to run the tests in a non-test ENV 🤷‍♂️

[BrandonShar]

This is awesome! I haven't used devcontainers yet, but I was actually planning on testing some with Claude Code next week. If no one else tests it first, I'll use this branch to try it out. @bknoles are you guys using devcontainers right now?

[bknoles]

We've had devcontainers in place on some Python work, but we use a dip/Ruby on Whales setup for Rails apps without any additional IDE config. I'd like to test this out as well! Making it easy to get a full environment up and running is definitely a worthy goal. The (non containerized) setup from the generators makes it so, so easy to tinker with stuff.