Github action for updating airflow

[greenape]

I think it would be nice to have a scheduled GH action which checked the current version of airflow, and if > than .airflow-version, branch, updated .airflow-version, and also update our corresponding Dockerfile, then open a PR.

Presumably this would also entail making the Dockerfile depend on .airflow-version.

[jc-harrison]

Would need to query Docker Hub's API to find the corresponding image digest, I guess.

[greenape]

Maybe one could invert it a little and have an action triggered by dependabot prs on the docker image that augments the pr from dependabot?