Source image xxx.dkr.ecr.us-east-1.amazonaws.com/aws-cdk/assets:xxx does not exist. Provide a valid source image.

### Before opening, please confirm:

- [X] I have installed the latest version of the Amplify CLI (see above), and confirmed that the issue still persists.
- [X] I have [searched for duplicate or closed issues](https://github.com/aws-amplify/amplify-cli/issues?q=is%3Aissue+).
- [X] I have read the guide for [submitting bug reports](https://github.com/aws-amplify/amplify-cli/blob/master/CONTRIBUTING.md#bug-reports).
- [X] I have done my best to include a minimal, self-contained set of instructions for consistently reproducing the issue.
- [X] I have removed any sensitive information from my code snippets and submission.

### How did you install the Amplify CLI?

sudo npm

### If applicable, what version of Node.js are you using?

v16.14.0

### Amplify CLI Version

7.6.23

### What operating system are you using?

Ubuntu

### Did you make any manual changes to the cloud resources managed by Amplify? Please describe the changes made.

No.

### Amplify Categories

custom

### Amplify Commands

push

### Describe the bug

Running `amplify push` with a custom resource that uses [`lambda.DockerImageCode.fromImageAsset`](https://docs.aws.amazon.com/cdk/api/v1/docs/@aws-cdk_aws-lambda.DockerImageCode.html#static-fromwbrimagewbrassetdirectory-props) fails with the titular error message.

### Expected behavior

The docker image should build and then be uploaded to ECR, then the lambda deployed.

### Reproduction steps

1. `amplify add custom`
2. Select CDK
3. In the CDK stack, add a function that points to a Dockerfile:
```ts
const fn = new lambda.DockerImageFunction(this, 'someLambda', {
code: lambda.DockerImageCode.fromImageAsset(path.resolve(__dirname, '../path/to/docker')),
});
```
4. `amplify push`
5. You will get the error: Source image xxx.dkr.ecr.us-east-1.amazonaws.com/aws-cdk/assets:xxx does not exist. Provide a valid source image.

### GraphQL schema(s)

n/a

### Log output

```
# Put your logs below this line

```

### Additional information

I assume this relates to the custom StackSynthesizer used by AWS Amplify, because it seems unable to handle CDK assets in the proper way.