Multiple Projects in Solution with .Net SDK Build

Hi, I am hoping you can help.

I have 3 projects in my solution. 2 projects that contain .net core apis that I wish to push to ACA and a 3rd common project that are referenced by both. However, I cant work out the correct syntax to use build and deploy one of these apis to ACA.
I have tried to set the
1-To set the appSourcePath to be the root of the solution but got the below error
Error: Ambiguity in selecting a project to build. Found multiple projects
2-To set the appSourcePath to be the folder containing one of the projects I want to build as a container but got an error as the
common .csproj could not be found
Skipping project "/xxx.csproj" because it was not found
error CS0234: The type or namespace name 'xxx' does not exist in the namespace 'xxxx' (are you missing an assembly reference?)
3-To set the dockerfilePath to be the folder containing the project that I wish to build into a container. However, not unexpectedly, I get an error that states that it is a directory and not a file. I am wanting to build without using a dockerfile and instead using a .net sdk build.

Hopefully, I am just missing something and you can point me in the right direction. Otherwise could I request an enhancement that a new input be added that works like the dockerfilePath so that I can define which project is to be built if there are multiple

Any help greatly received