Feature request: Provide option to customize the generated docker image tag format / prefix

### Describe your idea/feature/enhancement

I wish SAM CLI would generate image tag in the format of `--` rather than the current format - `--`. That way I can leverage the ECR image lifecycle to clean up obsolete images by specifying particular `tagPrefixList`. The current format doesn't apply tagPrefixList option.

E.g. Given the following metadata of the lambda function:
```
HelloWorldFunction:
Type: 'AWS::Lambda::Function'
Properties:
...
Metadata:
DockerContext: X/sam/test/hello_world
DockerTag: python3.9-v1
Dockerfile: Dockerfile
```

Generate docker tag as - `helloworldfunction-python3.9-v1-ffffff111111`

### Proposal

Option-1: Change the default format to `--`
Option-2: Provide additional configuration to specify the tag prefix in metadata. E.g. DockerTagPrefix: xxx
E.g.
```
HelloWorldFunction:
Type: 'AWS::Lambda::Function'
Properties:
...
Metadata:
DockerContext: X/sam/test/hello_world
Dockerfile: Dockerfile
DockerTagPrefix: myimageprefix
```

The above would generate docker tag as - `myimageprefix-ffffff111111`, disregard the DockerTag option. (DockerTag and DockerTagPrefix can be mutual-exclusive).

### Additional Details

Discussion chain in Premium Support CASE 10376090461