chore(project): cache docker image layers across CI jobs with buildx gha cache

[jaredlockhart]

Because

• setup-cached-build ran buildx with driver: docker and no cache-from/cache-to, so every CI job rebuilt megazords, schemas, cirrus and the experimenter images from scratch, duplicating the poetry/npm install and application-services fetch layers across the whole PR run.

This commit

• switches the builder to the docker-container driver and adds type=gha cache to every build the action drives, scoped per image.
• stands up a throwaway localhost:5000 registry and pushes megazords there so the container-driver builds can resolve the experimenter:megazords build context (previously satisfied from a locally loaded image under the daemon driver).
• parametrizes the megazords tag as MEGAZORD_IMAGE so local builds keep loading into the daemon while CI targets the registry.

This PR touches root/.github paths, so check-changed-paths runs the full CI suite here: the cache warms on this run and the payoff shows on subsequent runs of the same branch/base.

Fixes #16332

[jaredlockhart]

We tried this before but I wanted to try again and it still seems as if we lose more time on rehydrating the cache than it saves on build steps, which is annoying, so closing this.