cdk.lambda.DockerImageFunction: add ability to set image tag as parameter

### Describe the feature

On the lambda DockerImageFunction construct, it would be sweet if there was an ability to set the image tag.
for example:
```
const createKnowledgeIndexLambda = new lambda.DockerImageFunction(
this,
`CreateKnowledgeIndexLambda`,
{
code: lambda.DockerImageCode.fromImageAsset(
"lambda/knowledge/create_knowledge_index"
),
timeout: cdk.Duration.minutes(5),
imageTag: "latest"
}
);
```

### Use Case

When running several docker lambda functions in a stack or set of stacks, the amount of images starts to pile up in ECR because each image gets automatically tagged with a unique id.

Now I am aware that ECR retention policies can be created to automatically clear out old images but since these containers are tagged with unique IDs, it is difficult to create a policy that would, for example: clear out all images without a tag that are greater than 14 days old.

### Proposed Solution

Add the ability to set the imageTag as a parameter of the dockerImageFunction construct to something static/developer-defined. During deployment, set the image tag to the parameter defined in the construct.

From my knowledge of ECR, I believe this is how the workflow would play out:
- CDK deployment creates image with the tag defined in construct (eg latest)
- if there is an existing image tagged latest, ECR should automatically just untag that image
- This way we will be left with one image tagged latest and n images without a tag

I think that doing this would enable the ability to set a more straightforward ECR retention policy. Or hey honestly, it might just be easier/ better to just remove the old image if the new one builds successfully so this pile-up of images doesn't happen.

### Other Information

Definitely just taking a stab here at something I found a little annoying when working on some projects so I'm not sure if this was abstracted away or done for a reason someone much smarter than myself has a solid design decision around but either way wanted to throw it out there to see if we could get this on the map.

Thanks!

### Acknowledgements

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

### CDK version used

latest

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

macOS 13.0

Start with the lambda.DockerImageFunction construct and DockerImageCode.fromImageAsset entry points, then inspect how the deployment image is tagged in ECR. Done means a developer-defined imageTag can be supplied and the resulting tag and replacement behavior are covered by the relevant implementation tests.

Written by the indexing model from the issue text.