Dagger 0.8: Big Summer Clean Up

Dagger 0.8 introduces several breaking changes to its API, including the removal of the `Container.exitCode` field, which must now be retrieved with `ExecError` in the SDKs. This change affects how command execution status is checked and handled. Additionally, the `Host.envVariable` field is no longer supported, and other deprecated fields such as `Container.exec`, `Container.fs`, `Container.withFS`, and `Host.workdir` are removed or replaced with new alternatives. The Python SDK now requires the use of keyword arguments when they're optional, and the synchronous client has been removed in favor of an asynchronous client. These changes aim to improve readability, consistency, and performance with Dagger. As usual, it's recommended to consult the release notes and update instructions for a complete list of changes and updates.