Region hardcoded in Ghostscript layer: Comment includes fix/workaround about ps-serverless-app

[davidtucker]

This approach certainly works if you do intend to use a different region. I planned to have learners just use the regions I was specifying to reduce complexity, but this will work for anyone wanting to do something different.

[mike-whittaker-work]

This is a fix that I came up with - not an experienced TS dev so pls forgive any non-idiomatic code!
It just substitutes the default region in effect during the CDK build:

`

    const useDefaultRegion = (resource_arn:string): string => {
      const components: ArnComponents = cdk.Arn.split(resource_arn, cdk.ArnFormat.COLON_RESOURCE_NAME);
      const new_components: ArnComponents = {
        ...components,
        region: process.env.CDK_DEFAULT_REGION
      };
      return cdk.Arn.format(new_components);
    }

// and so, where originally there was the hardcoded resource, can now use

const createThumbnail = new NodejsServiceFunction(this, 'ThumbnailLambda', {
  entry: path.join(__dirname, '../../../services/processing/thumbnail.js'),
  timeout: cdk.Duration.seconds(120),
  layers: [
    lambda.LayerVersion.fromLayerVersionAttributes(this, 'GhostscriptLayerVersion', {
      layerVersionArn: useDefaultRegion('arn:aws:lambda:us-east-2:764866452798:layer:ghostscript:8'),
      compatibleRuntimes: [lambda.Runtime.NODEJS_14_X],
    }),
  ],
});

`