--output flag does not appear to be implmented

e.g.
```
simon@batou ~/d/p/terraform (lightsail_poc) [255]> aws lightsail push-container-image \
--profile techcon \
--service-name podcast-container-service \
--label podcast \
--image podcast:latest \
--output json
0e0cb3bb076c: Layer already exists
..
78561cef0761: Layer already exists
Digest: sha256:c594e64b6a7c0aa52342ffa610d3c522a299b37e468a0c284ee9b9f53d36c0d4
Image "podcast:latest" registered.
Refer to this image as ":podcast-container-service.podcast.1" in deployments.
```

In addition if I try and query a path with "---output json --query image", I get:
```
Object of type ParsedResult is not JSON serializable
```

I want to fish out the image name to use in deployments, at the moment I am doing this via a regex from the text output.