fix(lambda): fromDockerBuild output is located under /asset

Ensure `imagePath` ends with `/.` so that the content at that location
is copied.

See https://docs.docker.com/engine/reference/commandline/cp/

Closes #13439

Author: jogold

packages/@aws-cdk/aws-lambda/lib/code.ts

@@ -67,9 +67,11 @@ export abstract class Code {
    * @param options Docker build options
    */
   public static fromDockerBuild(path: string, options: DockerBuildAssetOptions = {}): AssetCode {
+    const imagePath = options.imagePath ?? '/asset';
     const assetPath = cdk.DockerImage
       .fromBuild(path, options)
-      .cp(options.imagePath ?? '/asset', options.outputPath);
+      // ensure imagePath ends with /. to copy the **content** at this path
+      .cp(imagePath.replace(/(\/|\/\.)?$/, '/.'), options.outputPath);
     return new AssetCode(assetPath);
   }
 

packages/@aws-cdk/aws-lambda/test/code.test.ts

@@ -346,7 +346,7 @@ describe('code', () => {
       // then
       expect(stack).toHaveResource('AWS::Lambda::Function', {
         Metadata: {
-          [cxapi.ASSET_RESOURCE_METADATA_PATH_KEY]: 'asset.38cd320fa97b348accac88e48d9cede4923f7cab270ce794c95a665be83681a8',
+          [cxapi.ASSET_RESOURCE_METADATA_PATH_KEY]: 'asset.20276d7803bfd6f4a2a5dc48692eb300d676b2a0dfdd4bda78c56c7c2c461515',
           [cxapi.ASSET_RESOURCE_METADATA_PROPERTY_KEY]: 'Code',
         },
       }, ResourcePart.CompleteDefinition);

packages/@aws-cdk/aws-lambda/test/docker-build-lambda/Dockerfile

@@ -1,3 +1,3 @@
 FROM public.ecr.aws/amazonlinux/amazonlinux:latest
 
-COPY index.js /asset
+COPY index.js /asset/