RuntimeDependenciesLayer rebuilding on every API request

First of all, thank you @Envek and others for both the great writeup at https://evilmartians.com/chronicles/serverless-typescript-a-complete-setup-for-aws-sam-lambda and for this template repo.

This is my first time using AWS SAM in general, and it's been tricky with Typescript.

## Problem

When running `sam local start-api`, every request I make to http://127.0.0.1:3000/ triggers a rebuild of the dependency layer.

## Steps to reproduce

- Clone fresh copy of repo and walk through the setup
- `sam local start-api`
- Visit http://127.0.0.1:3000/

The first visit will build the image as expected.

When refreshing the browse, based on [this comment](https://github.com/Envek/aws-sam-typescript-layers-example/blob/master/Makefile#L24), I would expect the image not to rebuild since no source code would have changed.

However, on each request, the image appears to be rebuilt with the following log line:

```
RuntimeDependenciesLayer is a local Layer in the template
```

The time it takes to rebuild this image makes it difficult to develop my frontend.

The workaround I have right now is to just point my frontend at my `dev` environment in AWS, but I'd prefer to be able to run both front & backend locally.