fix(ecr-assets): cannot build ARM images using modern stack synthesis (…

…#20563)

#20439 added support for the --platform option for docker build but the platform prop was not properly passed through when using the new DefaultStackSynthesizer. 

----

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### New Features

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packages/@aws-cdk/aws-ecr-assets/test/image-asset.test.ts

@@ -170,6 +170,22 @@ describe('image asset', () => {
     expect(assetMetadata && (assetMetadata.data as cxschema.ContainerImageAssetMetadataEntry).platform).toEqual('linux/arm64');
   });
 
+  testFutureBehavior('with platform: default synth edition', flags, App, (app) => {
+    // GIVEN
+    const stack = new Stack(app, 'Stack', { synthesizer: new DefaultStackSynthesizer() });
+    // WHEN
+    const asset = new DockerImageAsset(stack, 'Image', {
+      directory: path.join(__dirname, 'demo-image'),
+      platform: Platform.LINUX_ARM64,
+    });
+
+    // THEN
+    const asm = app.synth();
+    const stackAssets = JSON.parse(fs.readFileSync(path.join(asm.directory, 'Stack.assets.json'), { encoding: 'utf-8' }));
+    const dockerImageAsset = stackAssets.dockerImages[asset.assetHash];
+    expect(dockerImageAsset.source.platform).toEqual('linux/arm64');
+  });
+
   testFutureBehavior('asset.repository.grantPull can be used to grant a principal permissions to use the image', flags, App, (app) => {
     // GIVEN
     const stack = new Stack(app);

packages/@aws-cdk/core/lib/stack-synthesizers/_asset-manifest-builder.ts

@@ -93,6 +93,7 @@ export class AssetManifestBuilder {
         dockerBuildTarget: asset.dockerBuildTarget,
         dockerFile: asset.dockerFile,
         networkMode: asset.networkMode,
+        platform: asset.platform,
       },
       destinations: {
         [this.manifestEnvName(stack)]: {