feat: optimize Dev Container features

[lsorber]

Changes:

1. Directly include devcontainers-cli as a Dev Container feature instead of npm.
2. Don't install the Dev Container CLI in test.sh since it will be available by default, and when working without Docker we shouldn't modify the user's system.
3. Replace docker-in-docker with docker-outside-of-docker for Python apps, which is what the devcontainers-cli feature uses too and should be more lightweight than a full install of Docker inside the container. EDIT: Does not work on macOS after testing locally.
4. Specify the Starship major version.
5. Don't set "remoteUser": "root" as that is the default.
6. Replace containerEnv with remoteEnv. This closes Prepend to PATH instead of overwriting it #282.