How should builds using multiple container images be attested?

[marcelamelara]

How about a build being constructed using multiple container images?
ex: https://cloud.google.com/build/docs/build-config-file-schema#build_steps

Originally posted by @thirumalareddym in #1115 (comment)

[marcelamelara]

Notes from 10/23 discussion with @paveliak :
Different types of composition:

• Composition of containers within containers
• Containers execute sequential steps: Define a workflow which runs multiple step. They are the ephemeral environments, but all running within the same virtual machine
• We should assume these containers run in VMs, which are not rebooted
• Some platforms introduce abstractions for when your execution includes multiple containers
• Do the containers become an input into the build?
• What if the unit of attestation includes multiple instances of a VM?
• Can we aggregate the attestations as they are generated? We can technically provide the attestations up front as the environment is created, but nesting of environments makes this harder.
• If you're using multiple images for the environment, multiple VSA will be provided as early as possible.

[paveliak]

@marcelamelara I am struggling with whether we need to include any wording about this issue into the track. When I think about the containers two scenarios come into mind:

• Container hosts a build environment (i.e. build executor runs inside the container)
• Containers are used as building blocks for the individual tenant build steps

Original ask is about the second scenario (which is what Google Cloud Build does). If we look at the example provenance of Cloud Build then it treats those multiple container images as dependencies and includes them into the provenance according to the Build track. And so multiple container issue is a Build track concern.

A more interesting scenario is attesting container environments when build executor runs inside the container (actions-runner-controller could be an example of such a build platform). And I think we need to provide guidelines/requirements for the containers, which could be quite interesting at L3 level. But that probably needs to be tracked by a separate issue.