Update .NET Docker images for net5.0

[#72]

src/steeltoe-3.0.x--net5.0--webapi--csharp/Dockerfile

@@ -1,8 +1,8 @@
-FROM mcr.microsoft.com/dotnet/core/aspnet:3.1 AS base
+FROM mcr.microsoft.com/dotnet/core/aspnet:5.0 AS base
 WORKDIR /app
 EXPOSE 80
 
-FROM mcr.microsoft.com/dotnet/core/sdk:3.1 AS build
+FROM mcr.microsoft.com/dotnet/core/sdk:5.0 AS build
 WORKDIR /src
 COPY ["{{Namespace}}.csproj", "."]
 RUN dotnet restore "{{Namespace}}.csproj"