Refactor Dockerfile when airbyte-ci no longer requires rigid file structure

We need to revisit the Dockerfile in [PR #90](https://github.com/airbytehq/airbyte-python-cdk/pull/90) to clean up the workaround implemented due to airbyte-ci requiring a rigid file structure and invocation method.

This issue is a follow-up to the discussion in [this comment](https://github.com/airbytehq/airbyte-python-cdk/pull/90#discussion_r1866253308) by @aaronsteers.