Add Devcontainer Setup for Local Development

[elalemanyo]

This PR replaces the initial devcontainer proposal (#2120) with a much simpler approach that reuses the existing Dockerfile and delegates setup to bin/setup (thanks to the suggestion of @flavorjones).
The goal is the same: optional, zero-friction onboarding for new contributors, without changing the existing workflow for current devs.

Key points:

• builds from the repo’s Dockerfile (no separate dependency list to maintain)
• runs bin/setup inside the container as the main setup step
• keeps the devcontainer fully optional; existing local workflows stay unchanged

Context:

• This follows the feedback from Add Devcontainer Setup for Local Development #2120 about minimizing duplication and relying on the current build setup.
• This project is a gem for the Rails community, and making clone → open → run as easy as possible should help new developers get started quickly.