Built-in container images

**What I'd like:**
I'd like to include large container images (it could be 10GB+) into the AMI image. I am using warm pool to reduce time to pull, but still it costs traffic. It'd be best to include them in AMIs.

**Any alternatives you've considered:**
I have modified the source code for buildsys to be able to `docker-pull` and `docker-save` at build time, and added a package refers to `[[package.metadata.build-package.external-container-images]]`. Then, the package.spec copies the tar file and a service spec to execute `ctr -n k8s.io images import /path/to/tarfile` on boot. If wanted, I have willing to submit a PR.