Image build can fail as `cache warm` depends on nine external fetches with no retry

[alexhambley]

Issue

• Integration tests for Production-readiness refactor: drop MinIO, make Crate IDs more robust, add async validation and offline cache #186 failed as the Docker image build died in the cache-warm step.
• Failed run: https://github.com/eScienceLab/RO-Crate-Validation-Service/actions/runs/29488412430
• Identical re-run passed. rocrate-validator cache warm --all-profiles downloads nine profile vocabularies externally at build time and exits non-zero if any fetch fails.
• w3id.org drops connections occasionally, so every image build has a small chance of failing for reasons unrelated to the change being built.

Options

1. Retry loop in the Dockerfile in/around cache warm (e.g. 3 attempts with backoff)
2. Retry support upstream in rocrate-validator - this might be a better home for the fix