Hot reloading

### Describe your idea/feature/enhancement
We use SAM CLI to run lambda's successfully with the following command - `sam local start-api --template samtemplate.yml --env-vars env.json --skip-pull-image --warm-containers EAGER`. Using this command mitigates the per request building of the image/ container but still hot reloading is considerably slow and takes 10s+ for every change saved.
I wish SAM CLI would handle this in a faster way.

### Proposal
Changes in code can be applied to the container using volumes with docker -v command, this could be a solution to this issue if none other exists.
Things to consider:
1. Will this require any updates to the [SAM Spec](https://github.com/awslabs/serverless-application-model)