The Bazel ecosystem builds container images not by using Dockerfile, which contains non-reproducible primitives such as RUN and others. We do it by actually constructing the file trees and tarballs manually, then using them to compose the JSON manifest and indices. This is done via smaller hermetic and reproducible Bazel actions and thus enables the ecosystem to scale way beyond what alternative BuildKit-based solutions can.

https://www.youtube.com/watch?v=biYXmAv4Ppk&t=314s should be a good talk to study up on the matter. The speaker is now working at Apple.