.net core - build error with docker compose

I try to create a release build from my backend project with the Visual Studio, but I get the error message

Service 'backend_api' failed to build: The command '/bin/sh -c dotnet build ...

With "docker-compose build" I get the same error.

In Debug-Mode of the Visual Studio it works.

Dockerfile:

FROM mcr.microsoft.com/dotnet/core/aspnet:3.0-buster-slim AS base
WORKDIR /app
EXPOSE 80

FROM mcr.microsoft.com/dotnet/core/sdk:3.0-buster AS build
WORKDIR /src
COPY ["backendAPI/backendAPI.csproj", "backendAPI/"]
RUN dotnet restore "backendAPI/backendAPI.csproj"
COPY . .
WORKDIR "/src/backend_API"
RUN dotnet build "backendAPI.csproj" -c Release -o /app/build

FROM build AS publish
RUN dotnet publish "backendAPI.csproj" -c Release -o /app/publish

FROM base AS final
WORKDIR /app
COPY --from=publish /app/publish .
ENTRYPOINT ["dotnet", "backendAPI.dll"]

docker-compose.yml

version: '3.7'

services:
  backend_sql:
	image: mcr.microsoft.com/mssql/server:2017-GA-ubuntu
	container_name: testsql
	networks:
	  - test_network
	ports:
	  - "1440:1433"
	volumes:
	 - testsqldata:/var/opt/mssql

  backend_api:
	image: ${DOCKER_REGISTRY-}backendapi
	container_name: testapi
	build:
	  context: .
	  dockerfile: backendAPI/Dockerfile
	networks:
	  - test_network
	depends_on:
	 - backend_sql
	ports:
	 - "5000:80"

networks:
  test_network:
	external:
		name: test_network

volumes:
  testsqldata:
	driver: local
	name: testsqldata

Error-Message:

...
2>Step 7/16 : RUN dotnet restore "backendAPI/backendAPI.csproj"
2> ---> Running in ee20ee70cedf
2>  Restore completed in 41.8 sec for /src/backendAPI/backendAPI.csproj.
2>Removing intermediate container ee20ee70cedf
2> ---> 6cafbb7f9daf
2>Step 8/16 : COPY . .
2> ---> 74dc0dfe8145
2>Step 9/16 : WORKDIR "/src/backend_API"
2> ---> Running in e198c3ffb356
2>Removing intermediate container e198c3ffb356
2> ---> 114bfbba3151
2>Step 10/16 : RUN dotnet build "backendAPI.csproj" -c Release -o /app/build
2> ---> Running in 3e7bf02253ce
2>Microsoft (R) Build Engine version 16.3.2+e481bbf88 for .NET Core
2>Copyright (C) Microsoft Corporation. All rights reserved.
2>MSBUILD : error MSB1009: Project file does not exist.
2>Switch: backendAPI.csproj
2>Service 'backend_api' failed to build: The command '/bin/sh -c dotnet build "backendAPI.csproj" -c Release -o /app/build' returned a non-zero code: 1
2>C:\Program Files (x86)\Microsoft Visual Studio19\Professional\MSBuild\Sdks\Microsoft.Docker.Sdk\build\Microsoft.VisualStudio.Docker.Compose.targets(492,5): error DT1001: Service 'backend_api' failed to build: The command '/bin/sh -c dotnet build "backendAPI.csproj" -c Release -o /app/build' returned a non-zero code: 1
2>Done building project "docker-compose.dcproj" -- FAILED.

> "error MSB1009: Project file does not exist":

This error indicates that there's no such a *.proj file in current location. That happens because you spelled the path incorrectly: it should be backendAPI instead of backend_API. To fix that issue, change your code as below:

<pre>
FROM mcr.microsoft.com/dotnet/core/sdk:3.0-buster AS build
WORKDIR /src
COPY ["backendAPI/backendAPI.csproj", <b>"backendAPI/"</b>]
RUN dotnet restore "backendAPI/backendAPI.csproj"
COPY . .
<strike><b>WORKDIR "/src/backend_API"</b></strike>
<b>WORKDIR "/src/backendAPI"</b>
RUN dotnet build "backendAPI.csproj" -c Release -o /app/build
</pre>