Update Dockerfile to .NET 8 base images

Dockerfile

@@ -1,5 +1,5 @@
 # https://hub.docker.com/_/microsoft-dotnet
-FROM mcr.microsoft.com/dotnet/sdk:7.0 AS build
+FROM mcr.microsoft.com/dotnet/sdk:8.0 AS build
 WORKDIR ./sources
 
 # copy everything else and build app
@@ -8,7 +8,7 @@ WORKDIR ./sources/WebGoat.NET
 RUN dotnet publish -c release -o /app 
 
 # final stage/image
-FROM mcr.microsoft.com/dotnet/aspnet:7.0
+FROM mcr.microsoft.com/dotnet/aspnet:8.0
 WORKDIR /app
 COPY --from=build /app ./