DockerImageAsset.buildSsh should allow arrays

### Describe the feature

It's ok to have multiple `--ssh` arguments to pass multiple named keys.

### Use Case

Check out multiple private repositories on github using deployment keys. A single key cannot be authorized on multiple repos.

### Proposed Solution

_No response_

### Other Information

_No response_

### Acknowledgements

- [ ] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

### CDK version used

2.188.0

### Environment details (OS name and version, etc.)

linux

Locate the DockerImageAsset.buildSsh implementation and its tests, then inspect how --ssh arguments are assembled. Done means multiple named keys produce multiple --ssh arguments while the existing single-key behavior continues to work.

Written by the indexing model from the issue text.