Support for Mocking functions with layers and Docker

>**Note**: If your feature-request is regarding the AWS Amplify Console service, please log it in the
[official AWS Amplify Console forum](https://forums.aws.amazon.com/forum.jspa?forumID=314&start=0)

**Is your feature request related to a problem? Please describe.**
When I try to test layers locally I've seen errors which shows that it's not implemented yet. It would be nice if we could install yumda packages using Docker.
```
Unable to mock function. Mocking a function with layers is not supported. To test in the cloud: run "amplify push" to deploy your function to the cloud and then run "amplify console function" to test your function in the Lambda console.
```

**Describe the solution you'd like**
Use Docker to install yumda packages and test it locally, maybe with a flag `--docker`

**Describe alternatives you've considered**
Test locally using common mocks