Add a docker compose configuration file

[brettcannon]

As suggested by @malemburg in https://discuss.python.org/t/what-do-people-want-to-see-to-make-supporting-wasi-easier/94472/ , we could add a .devcontainer/compose.yml file to provide another way to get the dev container beyond the instructions in the devguide (which I'm improving). It would probably be something along the lines of:

# Start: docker compose --file .devcontainer/compose.yml up --detach
# Get a shell: docker compose exec cpython-dev bash
# Stop: docker compose down
# Cleanup: docker compose rm cpython-dev
services:
  cpython-dev:
    image: ghcr.io/python/devcontainer:latest
    container_name: cpython-dev
    restart: unless-stopped
    entrypoint: sleep 86400
    working_dir: /workspace
    volumes:
      - ../:/workspace

The biggest benefit I can see from other approaches to using the image outside of GitHub Codespaces like docker run is you don't need to know where the container registry is to get going.

[brettcannon]

And if you want an out-of-tree config extending the config above:

services:
  out-of-tree:
    extends:
      file: compose.yml
      service: cpython-dev
    working_dir: /build
    volumes:
    - ../../build:/build
    - ../:/workspace:ro

The problem with this is if ../../build doesn't exist then it creates it as root when docker compose up runs.