Task: add devcontainer to ease dev

Add a .devcontainer/Dockerfile with the dev environment. So we can spin a local dev container and/or codespaces. With that folks trying to contribute don't need to run the environment locally.

Create .devcontainer/Dockerfile for the Ruby development environment. Start by reviewing the repository's existing setup to identify what the container must provide, then verify that a local dev container or Codespace can start successfully and support contribution work.

Written by the indexing model from the issue text.