airflow dockerfile: set PYTHONPATH to DAGs folder

[tal66]

Fixes

Fixes WordPress/openverse#1564 by @AetherUnbound

Description

Adds the DAGs folder to the PYTHONPATH within the Docker container.

Testing Instructions

Per WordPress/openverse#1564, run:

1. just shell
2. python -c "from common import storage"
3. Observe that no ModuleNotFoundError exception is raised.

Checklist

• My pull request has a descriptive title (not a vague title like Update index.md).
• My pull request targets the default branch of the repository (main) or a parent feature branch.
• My commit messages follow best practices.
• My code follows the established code style of the repository.
• I added or updated tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no visible errors.

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[AetherUnbound]

Thanks for the contribution @tal66! I've edited your PR description so it aligns with our template. I'm having some trouble testing this locally, but I have a feeling it's something on my end.

[AetherUnbound]

I'm going to rebase this to include the changes from #502 and make sure it still passes CI

[AetherUnbound]

Looks good to me, works great with the test commands!

[zackkrida]

Hi @tal66 if you ever want to chat about Openverse and contributing more, we have a community Slack channel (#openverse) in the Making WordPress Slack Instance. You can learn how to join here, if you're interested:

https://make.wordpress.org/chat/