To have the SAM layer cache, compatible with the docker layer cache

### Describe your idea/feature/enhancement

When running `sam build` and `docker build`, they both have their own layer caches. If you edit your Dockerfile half way down and run `sam build`, when you go to run `docker build`, it starts from half way down instead of re-using the same layers.

Just having one-way compatibility from docker to SAM would solve my use case. You could `docker pull` the image from the last pipeline run into your codebuild, then `sam build` could use whatever layers hasn't changed instead of rebuilding everything from scratch.

### Proposal

To have sam build, able to use layers from images pulled through docker.

Things to consider:
[ ] The [SAM documentation](https://docs.aws.amazon.com/serverless-application-model/latest/developerguide/sam-specification.html) will need to be updated