This commit adds support for GitHub Codespaces. It includes a
devcontainer configuration that will be used to create a Codespace for
this repository. The configuration includes:

- Dockerfile: Installs `ruby:dev-3.2-buster` image. When the version of
  ruby used by this project changes, the Dockerfile should be
  updated. An attempt to install the correct version of ruby will be
  done, but it will take a while to download and compile the version.
- devcontainer.json: Specifies the configuration for the Codespace. It
  includes the Dockerfile to use, the extensions to install, and the
  settings to use.
- docker-compose.yml: Specifies the services to run in the Codespace. It
  includes the `postgres` service.
- post-create.sh: Script that runs after the Codespace is created. It
  optionally installs a new version of ruby, sets up the database, and
  prepares the environment.

…er-updates