test: add docker based spport and start adding tests

[asraa]

This PR adds the docker-based workflow to the supported trusted builders.

We currently only have main released for it, so I've began by only adding main tests. Follow-up here: #485

This is also gated behind SLSA_VERIFIER_EXPERIMENTAL due to the bundle format, anyway.

[laurentsimon]

want to create an issue tracking that the verification will need to be updated to verify @<tag> and not @main?

[laurentsimon]

want to create an issue tracking that the verification will need to be updated to verify @<tag> and not @main?

Ignore the comment, just saw the existing issue in the description.

[laurentsimon]

We currently only have main released for it

these test will need to be updated in the follow-up PR that enforces validation of the builder tag, correct?

[asraa]

these test will need to be updated in the follow-up PR, correct?

Yes! Right now this unblocks us from using verify in the e2e package repo though, so that's the idea. Without that we can't test @main and actually GA a release.