Dotnet Cannot Restore Project Correctly While Using Docker

I have a .Net webapi project, and I want to dockerize it. Without docker, dotnet sdk(6.0) can restore everything correctly and there is no obstacle for me to publish the app. But when I use docker to do that, it gives me tones of errors with the error codes CS0246, CS8714, CS0103, CS0535, CS0738. You can find my project folder structure and relevant information below:

    src
     |-->API
     |    |-->API.csproj
     |    |-->Some other stuff
    tools
     |-->Docker
     |    |-->Dockerfile
    docker-compose.yml

#./src/Docker/Dockerfile

FROM mcr.microsoft.com/dotnet/sdk:6.0 AS build
WORKDIR /source

COPY *.sln ./
COPY ./src/API src/API

WORKDIR /src/API
RUN dotnet restore
RUN dotnet publish -c release -o /src/API

FROM mcr.microsoft.com/dotnet/aspnet:6.0
WORKDIR /src/API
COPY --from=build /src/API ./
EXPOSE 80/tcp
ENV ASPNETCORE_URLS http://*:80
ENTRYPOINT ["dotnet", "API.dll"]

#./docker-compose.yml

version: '3'

services:
  api:
    build:
      context: .
      dockerfile: tools/Docker/Dockerfile
    ports:
      - "5009:5009"
    environment:
      ASPNETCORE_ENVIRONMENT: "prod"