How to specify image name in sam build with template containing multiple functions

I've had a look through the extensive documentation for AWS SAM and which CLI parameters are supported for the `sam build` command but I couldn't see any option to specify the name of the docker image that gets built? I am already using the `Metadata` section with the `DockerTag` option to specify the tag but I'd like to specify the image name.

I have two Functions defined which are built as a single Docker image which builds and runs perfectly fine, however, when I run a `sam build` it seems to just select the Logical Resource ID of the first `AWS::Serverless::Function` defined in the template.

Here is my template:

```
Transform: AWS::Serverless-2016-10-31

Parameters:
DockerTag:
Type: String
Default: local-dev

Resources:
TestFunction1:
Type: AWS::Serverless::Function
Properties:
FunctionName: TestFunction1
PackageType: Image
ImageUri: !Sub 'testfunction1:${DockerTag}'
ImageConfig:
Command:
- TestLambdas::TestLambdas.TestFunction1::Invoke
Role: !Sub 'arn:aws:iam::${AWS::AccountId}:role/lambda-execution-role'
MemorySize: 256
Timeout: 30
Events:
ScheduledEvent:
Type: Schedule
Properties:
Schedule: rate(3 minutes)
Name: test-schedule-rule
Enabled: true
Metadata:
DockerTag: !Ref DockerTag
DockerContext: .
Dockerfile: ./Dockerfile

TestFunction2:
Type: AWS::Serverless::Function
Properties:
FunctionName: TestFunction2
PackageType: Image
ImageUri: !Sub 'testfunction1:${DockerTag}'
ImageConfig:
Command:
- TestLambdas::TestLambdas.TestFunction2::Invoke
Role: !Sub 'arn:aws:iam::${AWS::AccountId}:role/lambda-execution-role'
MemorySize: 256
Timeout: 30
Events:
ScheduledEvent:
Type: Schedule
Properties:
Schedule: rate(3 minutes)
Name: test-schedule-rule
Enabled: true
Metadata:
DockerTag: !Ref DockerTag
DockerContext: .
Dockerfile: ./Dockerfile
```

And here is the output of running `sam build` (Notice this one image being built contains two functions)

```
PS C:\Workspace\Lambda.Handlers\Lambda.Handlers.Test> sam build
Building codeuri: C:\Workspace\Lambda.Handlers\Lambda.Handlers.Test runtime: None metadata: {'DockerTag': 'local-dev', 'DockerContext': 'C:\\Workspace\\Lambda.Handlers\\Lambda.Handlers.Test', 'Dockerfile': './Dockerfile'} functions: ['TestFunction1', 'TestFunction2']
Building image for TestFunction1 function
Setting DockerBuildArgs: {} for TestFunction1 function
Step 1/3 : FROM public.ecr.aws/lambda/dotnet:5.0
---> 41fed005ba97
Step 2/3 : COPY ./* /var/task/
---> Using cache
---> 6fac8005ce03
Step 3/3 : ENTRYPOINT ["/lambda-entrypoint.sh"]
---> Using cache
---> d691b293f1c1
Successfully built d691b293f1c1
Successfully tagged testfunction1:local-dev <--------- I WOULD LIKE TO SET testfunction1 TO SOMETHING ELSE

Build Succeeded

Built Artifacts : .aws-sam\build
Built Template : .aws-sam\build\template.yaml

Commands you can use next
=========================
[*] Invoke Function: sam local invoke
[*] Deploy: sam deploy --guided
```

If I re-order `TestFunction1` and `TestFunction2`, then the built image is then named `testfunction2`.

Looking forward to any help/advice if what I'm trying to achieve is even possible...

Thanks