copilot fails with "templates/custom-resources/nlb-cert-validator.js: file does not exist" (on Macbook Pro M1)

I'm experimenting with AWS Copilot. I have an existing trivial nodejs project that has already been containerized with Docker. I'm trying to use it with AWS Copilot to get a feel for copilot. Note I am running this on a Macbook Pro M1, I wonder if this is an architecture issue or if that's a red herring...

The copilot init process fails (after doing a lot successfully) with:

 => [run 1/1] RUN yarn build 28.2s
 => exporting to image 5.1s
 => => exporting layers 5.1s
 => => writing image sha256:260f1aae6a9d11b023672a1f3753161d1d38afa118c483804f9ef79d0c4e52a1                                                               0.0s
 => => naming to xxx.dkr.ecr.us-east-2.amazonaws.com/em-sandbox/em-nodejs-service-template                                                        0.0s

Use 'docker scan' to run Snyk tests against images to find vulnerabilities and learn how to fix them
Login Succeeded
Using default tag: latest
The push refers to repository [xxx.dkr.ecr.us-east-2.amazonaws.com/em-sandbox/em-nodejs-service-template]
97b95ee3261b: Pushed
59618adc7d11: Pushed
4c3b5d9c5d94: Pushed
1c37a59f80eb: Pushed
099cd5535f68: Pushed
bb2ebe54bcf0: Pushed
cd2258b7ca68: Pushed
fbf63a621b59: Pushed
994393dc58e7: Pushed
latest: digest: sha256:2481df4d9803e63ddde3992a9224cda89e27365d859f627c962692436715e14a size: 2208
✘ upload deploy resources for service em-nodejs-service-template: read custom resources for a "Load Balanced Web Service": read custom resource NLBCertValidatorFunction at path custom-resources/nlb-cert-validator.js: read template custom-resources/nlb-cert-validator.js: open templates/custom-resources/nlb-cert-validator.js: file does not exist

For what it's worth, here's how I answered the initial questions in copilot init. I did also ask for a test environment to be set up.

Application name: em-sandbox
Workload type: Load Balanced Web Service
Service name: em-nodejs-service-template
Dockerfile: ./Dockerfile
Port: 4000
Ok great, we'll set up a Load Balanced Web Service named em-nodejs-service-template in application em-sandbox listening on port 4000.

✔ Proposing infrastructure changes for stack em-sandbox-infrastructure-roles
- Creating the infrastructure for stack em-sandbox-infrastructure-roles                         [create complete]  [40.8s]
  - A StackSet admin role assumed by CloudFormation to manage regional stacks                   [create complete]  [12.9s]
  - An IAM role assumed by the admin role to create ECR repositories, KMS keys, and S3 buckets  [create complete]  [14.3s]
✔ The directory copilot will hold service manifests for application em-sandbox.

Note: Architecture type arm64 has been detected. We will set platform 'linux/x86_64' instead. If you'd rather build and run as architecture type arm64, please change the 'platform' field in your workload manifest to 'linux/arm64'.
✔ Wrote the manifest for service em-nodejs-service-template at copilot/em-nodejs-service-template/manifest.yml
Your manifest contains configurations like your container size and port (:4000).

- Update regional resources with stack set "em-sandbox-infrastructure"  [succeeded]  [0.0s]
All right, you're all set for local development.
Deploy: Yes

✔ Wrote the manifest for environment test at copilot/environments/test/manifest.yml
...etc...