Fix: Update Dockerfile to target .NET 9.0

[Navaneethnanda]

This PR addresses the issue described in #3245.

Type of Issue: Bug

Current Behavior:
When building a Docker image following the instructions in
[ContainerReferenceServer.md](https://github.com/OPCFoundation/UA-.NETStandard/blob/master/Docs/ContainerReferenceServer.md),
the resulting image (or the one hosted on GitHub) fails to run.
This occurs because the Dockerfile currently uses unversioned base images:

FROM mcr.microsoft.com/dotnet/runtime AS base
FROM mcr.microsoft.com/dotnet/sdk AS build

These pull the latest .NET (currently 9.0) images, while the environment variables are still configured for .NET 8.0.
As a result, the container fails to run due to version mismatch.

Resolution:
The project has been tested and verified to be compatible with .NET 9.0.
The environment options in the Dockerfile have been updated to target .NET 9.0 accordingly.
This ensures compatibility with both .NET 9.0 and later versions.

Expected Behavior:
The container builds and runs successfully without errors.

Steps to Reproduce:

1. Build the Docker image using the existing instructions.

2. Observe that the container fails to start due to version mismatch.

3. With this fix applied, the container should run correctly using:

   FROM mcr.microsoft.com/dotnet/runtime:9.0 AS base
   FROM mcr.microsoft.com/dotnet/sdk:9.0 AS build
   

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