feat(docker): Add PYTHONUNBUFFERED to Dockerfile TASK-1303

[bufke]

🗒️ Checklist

1. run linter locally
2. update all related docs (API, README, inline, etc.), if any
3. draft PR with a title <type>(<scope>)<!>: <title> TASK-1234
4. tag PR: at least frontend or backend unless it's global
5. fill in the template below and delete template comments
6. review thyself: read the diff and repro the preview as written
7. open PR & confirm that CI passes
8. request reviewers, if needed
9. delete this section before merging

📖 Description

Default docker image to PYTHONUNBUFFERED mode as recommended for Docker environments

💭 Notes

Can replace setting PYTHONUNBUFFERED in kobo-helm-chart, kobo-docker, and kobo-compose. Anyone using the Dockerfile, is inherently using Docker and should set this.