build image also as unprivileged user

Dockerfile

@@ -1,10 +1,14 @@
 FROM microsoft/dotnet:2.0-sdk-stretch AS build-env
-WORKDIR /app
+
+RUN useradd -d /home/dotnet -ms /bin/bash dotnet
+USER dotnet
+
+WORKDIR /home/dotnet/app
 
 # Copy csproj and restore as distinct layers
 COPY ./src/Hamuste.csproj ./
-RUN dotnet restore && \
-    dotnet add package ILLink.Tasks -v 0.1.4-preview-981901 -s https://dotnet.myget.org/F/dotnet-core/api/v3/index.json
+RUN dotnet add package ILLink.Tasks -v 0.1.4-preview-981901 -s https://dotnet.myget.org/F/dotnet-core/api/v3/index.json &&\
+    dotnet restore
 
 # Copy everything else and build
 COPY ./src ./
@@ -16,5 +20,5 @@ RUN useradd -d /home/dotnet -ms /bin/bash dotnet
 USER dotnet
 WORKDIR /home/dotnet/app
 ENV ASPNETCORE_URLS=http://+:9999
-COPY --from=build-env /app/out ./
+COPY --from=build-env /home/dotnet/app/out ./
 ENTRYPOINT ["./Hamuste"]