(aws-ecr-assets): Dockerfile.dockerignore is not used

### What is the problem?

.. instead `.dockerfile` needs to be present. Docker it self supports `Dockerfile.dockerignore` - which is useful when -f (dockerfile prop) is given

### Reproduction Steps

```
new ecrAssets.DockerImageAsset(this, 'MyImage', {
directory: path.join(__dirname, '..', '..'),
});
```

### What did you expect to happen?

Give` -f MyDockerFile` (`dockerfile: MyDockerFile`) then still .`dockerignore` is needed

### What actually happened?

`MyDockerFile.dockerignore` is considered for exclusion

### CDK CLI Version

1.130.0

### Framework Version

_No response_

### Node.js Version

16.13.0

### OS

macOS

### Language

Typescript

### Language Version

4.5.2

### Other information

_No response_

Start at the aws-ecr-assets DockerImageAsset entry point and reproduce the case using the directory, dockerfile property, and MyDockerFile.dockerignore described in the issue. Trace how the Docker build context selects ignore files; done means the expected Dockerfile.dockerignore behavior is covered by a regression test.

Written by the indexing model from the issue text.