Deployment-related workflow improvements

[onyxfish]

Apologies for a grab bag, but this PR is a bunch fo small deployment-adjacent workflow improvements

• Added explicit documentation that this repository uses Github flow
• Added command to Dockerfile ensure apt dependencies are updated in the container at build time
• Added healthchecks to the Docker image and the ECS configuration (sharing a script)
• Configured CI to run on PR rather than push (to gate merges)
• Removed unused arguments from the CI configuration
• Added precommit hook for ruff and vulture

I also changed the following GitHub settings:

• Protected main branch
• Required 1 approval for merge
• Required squash merges for clean git history

Up next:

• Backport sceptre best practices for zephier-contrib-web #55