fix for Dockerfile platforms not working correctly

src/docker/Dockerfile-AasxServerAspNetCore

@@ -1,5 +1,5 @@
 # Use a multi-stage build to build and publish the .NET application
-FROM --platform=linux/amd64 mcr.microsoft.com/dotnet/sdk:8.0 AS build-env
+FROM --platform=$BUILDPLATFORM mcr.microsoft.com/dotnet/sdk:8.0 AS build-env
 WORKDIR /repo/src
 
 # Copy the source code and build
@@ -12,7 +12,7 @@ RUN dotnet build -o /out/AasxServerAspNetCore AasxServerAspNetCore -v d
 RUN dotnet publish -c Release -v d --no-restore
 
 # Use a runtime image to run the application
-FROM --platform=linux/amd64 mcr.microsoft.com/dotnet/aspnet:8.0 as base
+FROM --platform=$BUILDPLATFORM mcr.microsoft.com/dotnet/aspnet:8.0 as base
 EXPOSE 5001/tcp
 EXPOSE 5001/udp
 ENV ASPNETCORE_URLS=http://+:5001

src/docker/Dockerfile-AasxServerBlazor

@@ -1,6 +1,6 @@
 # Use a multi-stage build to build and publish the .NET application
 # Specify the initial base architecture (amd64 in this case)
-FROM --platform=linux/amd64 mcr.microsoft.com/dotnet/sdk:8.0 AS build-env
+FROM --platform=$BUILDPLATFORM mcr.microsoft.com/dotnet/sdk:8.0 AS build-env
 WORKDIR /repo/src
 
 # Copy everything else and build
@@ -14,7 +14,7 @@ RUN dotnet publish -c Release -v d --no-restore
 
 # Use a runtime image to run the application
 # Specify the initial base architecture (amd64 in this case)
-FROM --platform=linux/amd64 mcr.microsoft.com/dotnet/aspnet:8.0 as base
+FROM --platform=$BUILDPLATFORM mcr.microsoft.com/dotnet/aspnet:8.0 as base
 WORKDIR /repo/src
 RUN apt update && apt upgrade --yes
 RUN apt install -y curl nano libgdiplus