aws-ecr-assets: DockerImageAsset is unusable with Docker Buildkit builder, which is the default in recent Docker for Mac

### Describe the bug

BuildKit is the default builder on Docker for Mac and `DockerImageAsset` doesn't appear to set an output, so builds will fail as they aren't saved to the local docker registry (only build cache by default).

### Expected Behavior

DockerImageAsset should either configure `output=docker` or correctly use Buildkit to push to the ECR Repository when Buildkite is the default builder.

### Current Behavior

Building on a mac with a recent Docker for Mac (mine is 4.0.1), deploy will fail with this error:

> Error response from daemon: No such image: cdkasset-ba6f83a156c20286cbce8a4f5c837ca1a2a4e01d9b3de43cc6e9c2fa4a6e6fc0:latest

and a warning earlier that makes it clear the issue:
> time="2022-09-10T12:52:04+10:00" level=warning msg="No output specified for docker-container driver. Build result will only remain in the build cache. To push result image into registry use --push or to load image into docker use --load"

### Reproduction Steps

```
const image = new ecrAssets.DockerImageAsset(this, 'DockerImage', {
directory: path.join(__dirname, '/../docker/agent'),
});
```

### Possible Solution

This could possibly be worked around with [supporting `--output`](https://github.com/aws/aws-cdk/issues/20566), although given Buildkit is likely the future default, this seems like a bad option.

### Additional Information/Context

This is mentioned in https://github.com/aws/aws-cdk/issues/12472#issuecomment-904937818 and https://github.com/aws/aws-cdk/issues/20566, but neither of the comments make it clear that this is essentially a show stopper (without good workarounds) for anyone on a recent Docker for Mac.

### CDK CLI Version

2.39.1 (build f188fac)

### Framework Version

_No response_

### Node.js Version

v18.8.0

### OS

macOS 12.4

### Language

Typescript

### Language Version

_No response_

### Other information

_No response_

Start from the DockerImageAsset entry point and reproduce the asset build with Docker BuildKit enabled, checking how the build result is made available to the local Docker registry or pushed to ECR. Done means the reproduced deployment no longer fails with “No such image” and the BuildKit output warning is addressed.

Written by the indexing model from the issue text.