Streamline vulnerability scan workflow to build docker image once

[lisac]

Ticket

Implements a proposed improvement documented in workflow vulnerability-scans.yml:

NOTE: The workflow isn't able to pass the docker image between jobs, so each builds the image.
A future PR will pass the image between the scans to reduce overhead and increase speed

Changes

Refactors creation of the app's docker image in the vulnerability-scans.yml workflow:

• new job: build-and-cache: builds the image and puts it in a cache
• updates other jobs to use the cached image, instead of building the image as part of the job

Context for reviewers

This is taken from the implementation in HHS/simpler-grants-gov: https://github.com/HHS/simpler-grants-gov/blob/6dadc31f8474c386b824654d3ce2bb428bec584f/.github/workflows/vulnerability-scans.yml , which was authored by @daphnegold (thank you!).
Minor adjustment to use ubuntu-latest.

Testing

navapbc/platform-test#196

[lisac]

not ready for review