Set explicit arm64 platform on Docker image build (#30)

fromAsset() builds for the host architecture by default. On x86_64
GitHub Actions runners, this produces an x86 image even with QEMU
installed — the Fargate ARM64 task then fails with exec format error.

lib/constructs/socket-mode.ts

@@ -1,5 +1,6 @@
 import { Construct } from 'constructs';
 import * as ecs from 'aws-cdk-lib/aws-ecs';
+import * as ecr_assets from 'aws-cdk-lib/aws-ecr-assets';
 import * as ec2 from 'aws-cdk-lib/aws-ec2';
 import * as secretsmanager from 'aws-cdk-lib/aws-secretsmanager';
 import * as lambda from 'aws-cdk-lib/aws-lambda';
@@ -37,6 +38,7 @@ export class SocketModeConstruct extends Construct {
     taskDef.addContainer('socket-mode', {
       image: ecs.ContainerImage.fromAsset(
         path.join(__dirname, '../../services/socket-mode'),
+        { platform: ecr_assets.Platform.LINUX_ARM64 },
       ),
       environment: {
         APP_TOKEN_SECRET_ARN: appTokenSecret.secretArn,