Switch Heroku deployment to Ruby buildpack and isolate Docker for distribution

[tomohiro]

Background

Initially, this PR attempted to migrate the Heroku deployment to a Docker-based pipeline (container stack via heroku.yml). However, the ENTRYPOINT ["bundle"] in our Dockerfile—which is highly optimized for DockerHub distribution—conflicted with Heroku's internal container runtime wrapper (/bin/sh -c), causing startup crashes.

Solution

To keep the Dockerfile clean and pristine for future DockerHub/GHCR distribution without dirtying it with Heroku-specific workarounds, we changed our strategy to fall back to Heroku's standard Ruby Buildpack (heroku-26).

Changes

• .devcontainer/devcontainer.json: Added the docker-in-docker feature to support local Docker testing within Codespaces.
• Procfile: Re-introduced to explicitly define the web process command (bundle exec rackup) for the Heroku Buildpack.
• heroku.yml: Deleted, as we are no longer using Manifest-based Docker deploys on Heroku.
• app.json: Kept "stack": "heroku-26" to leverage the latest standard Heroku stack.

Testing

• Verified that the Heroku Review App builds successfully using the Ruby Buildpack and launches the Rack application without any /bin/sh or Bundler conflicts.