DigitalOcean App Platform | ASP.NET CORE MVC APP | ERROR: Build Error: Non-Zero Exit

Here my Dockerfile:

FROM mcr.microsoft.com/dotnet/core/sdk:3.1 AS build
WORKDIR /source

COPY mytestapp-Production-MVC.sln .
COPY mytestapp-Production-MVC/mytestapp-Production-MVC.csproj ./mytestapp-Production-MVC
COPY mytestapp-Production-MVC/ ./

# copy everything else and build app
# COPY mytestapp-Production-MVC/. ./source/mytestapp-Production-MVC
WORKDIR /source/mytestapp-Production-MVC
RUN dotnet restore
RUN dotnet publish -c release -o /app --no-restore

# final stage/image
FROM mcr.microsoft.com/dotnet/core/aspnet:3.1
WORKDIR /app
COPY --from=build /app ./
EXPOSE 80
ENTRYPOINT ["dotnet", "mytestapp-Production-MVC.dll"]

I get the following ERROR:

mytestapp-production-mvc-live | 16:44:03 INFO[0032] Running: [/bin/sh -c dotnet restore]  
mytestapp-production-mvc-live | 16:44:46 error building image: error building stage: failed to execute command: starting command: fork/exec /bin/sh: not a directory
auerdev-production-mvc-live | 16:44:46 exit status 1

I hope someone can help.

Greetings, Sven