[Tracking Issue] CI Improvements

[niccoloraspa]

Possible improvements for our current CI setup

• Improve build time (flashbox-l1 and flashbox-l2 are built in parallel right now but it could make sense to build them sequentially)
• Run image build only when changes are present
• Caching
• Upload artifacts to S3
• Calculate measurements
• Reproducibility tests as in trunk/buildernet
• Smoke tests on image