feat: add multistage release dockerbuilds

* this makes the whole thing self-sufficient - i.e. no more reliance on
  host's dotnet, everything is pulled during the build process

* makefile has been updated to reflect this change so that
  the 'dockerbuild' target is no longer dependent on 'releasebuild' target

Dockerfile

@@ -1,6 +1,16 @@
+FROM mcr.microsoft.com/dotnet/core/sdk:2.1-alpine
+
+COPY . /src
+WORKDIR /src
+RUN sed -i 's/2.1.519/2.1.811/' global.json
+RUN dotnet --version && dotnet --info
+RUN dotnet clean
+RUN dotnet restore
+RUN dotnet build -c Release
+
 FROM mcr.microsoft.com/dotnet/aspnet:2.1
 
-COPY bin/Release/netcoreapp2.1/publish/ App/
+COPY --from=0 /src/bin/Release/netcoreapp2.1/publish/ App/
 WORKDIR /App
 ENV ASPNETCORE_ENVIRONMENT=Release
 ENTRYPOINT ["dotnet", "pwt-0x01-ng.dll"]

Makefile

@@ -20,7 +20,7 @@ run:
 releasebuild: clean restore
 	$(CC) publish -c Release
 
-dockerbuild: releasebuild
+dockerbuild:
 	docker build -t $(dtag) -f $(dfile) .
 
 dockerdevbuild: