Fix Docker .NET version

Dockerfile

@@ -1,4 +1,4 @@
-FROM mcr.microsoft.com/dotnet/sdk:3.1 AS build-env
+FROM mcr.microsoft.com/dotnet/sdk:5.0 AS build-env
 WORKDIR /app
 COPY . ./
 RUN ./build.sh
@@ -6,7 +6,7 @@ RUN sed -i "s/<AuthEndpoint>/${AuthEndpoint}/g" output/appsettings.json
 RUN sed -i "s/<AuthApiKey>/${AuthApiKey}/g" output/appsettings.json
 RUN sed -i "s/<DatabaseConnectionString>/${ConnectionString}/g" output/appsettings.json
 
-FROM mcr.microsoft.com/dotnet/aspnet:3.1
+FROM mcr.microsoft.com/dotnet/aspnet:5.0
 WORKDIR /app
 COPY --from=build-env /app/output .
 ENTRYPOINT ["dotnet", "Robware.Api.Mailboxes.dll"]