Multiple containers are available since Serving v0.16 (serverless 1.10)  but the status is still "alpha" in upstream.

We need to explain it in our doc:

• The support policy of multiple containers in downstream. (Perhaps tech preview?)
• How to enable the alpha feature (you need to modify KnativeServing CR for config-features).

Additional resources

• Upstream docs issue [https://github.com/knative/docs/issues/2787]

• • PR: https://github.com/knative/docs/pull/2732 awaiting lgtm from SMEs
• Feature flags docs: https://knative.dev/docs/serving/feature-flags/#multi-containers