Consider disabling UseAppHost in generated Dockerfiles

[bwateratmsft]

From @mthalman originally in microsoft/DockerTools#321

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The Dockerfiles generated by VS include the following line:

RUN dotnet publish "WebApplication.csproj" -c Release -o /app/publish

This ends up outputting both a WebApplication executable and a WebApplication.dll library file in the output directory. Either file can be used to execute the app. The executable can be executed directly while the DLL requires the use of the .NET CLI (dotnet WebApplication.dll). From an image size perspective, it seems wasteful to include both files in the output. Only one of them is actually needed in order to execute the app. The Dockerfile is configured to only make use of the DLL:

ENTRYPOINT ["dotnet", "WebApplication31.dll"]

So it seems reasonable to simply not include the executable in the output.

By using the UseAppHost property, the publish logic can be configured to not include the executable in the output.

I propose that the Dockerfiles generated by the tools set this property by default:

RUN dotnet publish "WebApplication.csproj" -c Release -o /app/publish /p:UseAppHost=false

[bwateratmsft]

This is now released in Docker extension version 1.19.0.