dotnet build --no-restore fails in Docker

Steps to reproduce

Create a .NET Core 2.1 console app:

|-Dockerfile
|-Example1.csproj
|-Example1.sln
|-Program.cs

In Dockerfile restore packages as a first step, and the build with --no-restore as a second step:

FROM microsoft/dotnet:2.1-sdk
WORKDIR /app

# create build image: (restore as distinct layer to speed up build, so we can use --no-restore below)
COPY ./Example1.sln ./
COPY ./*.csproj ./
RUN dotnet restore 

# copy source code and build
COPY ./ ./
RUN dotnet build -c Release -o out --no-restore

Expected behavior

Project builds fine in Docker

Actual behavior

Build fails complaining about missing project.assets.json (notice /app/C:/Users/andrew/ in the path)

/usr/share/dotnet/sdk/2.1.403/Sdks/Microsoft.NET.Sdk/targets/Microsoft.PackageDependencyResolution.targets(198,5): error NETSDK1004: Assets file '/app/C:/Users/andrew/source/repos/Example1/obj/project.assets.json' not found. Run a NuGet package restore to generate this file. [/app/Example1.csproj]

Build FAILED.

Environment data

$ dotnet --info
.NET Core SDK (reflecting any global.json):
 Version:   2.1.403
 Commit:    04e15494b6

Runtime Environment:
 OS Name:     Windows
 OS Version:  10.0.17134
 OS Platform: Windows
 RID:         win10-x64
 Base Path:   C:\Program Files\dotnet\sdk\2.1.403\

Host (useful for support):
  Version: 2.1.5
  Commit:  290303f510

.NET Core SDKs installed:
  2.1.202 [C:\Program Files\dotnet\sdk]
  2.1.403 [C:\Program Files\dotnet\sdk]

.NET Core runtimes installed:
  Microsoft.AspNetCore.All 2.1.5 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.App 2.1.5 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 2.0.9 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.1.5 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]

To install additional .NET Core runtimes or SDKs:
  https://aka.ms/dotnet-download

[MichaelSimons]

@andreister, Can you provide a concrete set of steps that can be used to reproduce this issue? Is the directory you are using as you Docker context clean? Make sure you don’t have anything that is getting copied into the Docker image (via COPY ./ ./) that is causing this behavior. You may need to define a .dockerignore file to exclude these.

**/bin
**/obj
**/.vscode
**/.vs
...

[MichaelSimons]

@andreister - please refer to Docker’s documentation on the .dockerignore file