ci: use registry cache for container builds

[kajalj22]

What does this PR do ?

Switches the container build job from inline BuildKit cache to the reusable workflow's registry-backed cache. This lets fresh/ephemeral build runners reuse the expensive Docker layer that runs uv sync across dependency extras.

Issues

Linear: AUT-492

Usage

N/A - CI workflow change only.

Before your PR is "Ready for review"

Pre checks:

• Make sure you read and followed Contributor guidelines
• Did you write any new necessary tests? Workflow syntax validation only; no runtime code changed.
• Did you run the unit tests and functional tests locally? Visit our Testing Guide for how to run tests
• Did you add or update any necessary documentation? No docs needed for this CI-only change.

Additional Information

Validation run locally:

• python3 YAML parse/assertion for .github/workflows/cicd-main.yml
• git diff --check

actionlint and Docker are not installed in this cloud environment, so a full container build must run in CI.

Linear Issue: AUT-492

  

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