Run Snyk docker security scans nightly

[jackwotherspoon]

Add a Github Action to scan our published docker images nightly: https://github.com/snyk/actions/tree/master/docker

Action will notify the team if vulnerabilities have been found in any of the base images so that appropriate action (potential release with updated base image) can be taken.

[enocom]

This would be in addition to the container scanning we do in Artifact Registry.

[enocom]

Also, we should port this to AlloyDB Auth Proxy as well.

[enocom]

Cf. https://cloud.google.com/artifact-analysis/docs/os-scanning-on-demand