ENH: Add v6.0b01 dockcross image to GHCR mirror

Add docker.io/dockcross/manylinux_2_28-x64:20250913-6ea98ba used by
ITKPythonPackage v6.0b01 (currently only ITKTotalVariation).

Complete image tag coverage across all ITKPythonPackage versions:
- v5.4.0-v5.4.5: 20240304-9e57d2b (x64), 2024-03-25-9206bd9 (aarch64)
- v6.0b01: 20250913-6ea98ba (x64), 2025.08.12-1 (aarch64)
- v6.0b02/main: 20260203-3dfb3ff (x64), 2025.08.12-1 (aarch64)
- manylinux2014-x64: 20240304-9e57d2b (all versions)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hjmjohnson

.github/workflows/mirror-dockcross-images.yml

@@ -24,7 +24,10 @@ jobs:
             target: dockcross-manylinux2014-x64:20240304-9e57d2b
           - source: quay.io/pypa/manylinux_2_28_aarch64:2024-03-25-9206bd9
             target: dockcross-manylinux_2_28-aarch64:2024-03-25-9206bd9
-          # v6.0b02 image tags
+          # v6.0b01 image tags
+          - source: docker.io/dockcross/manylinux_2_28-x64:20250913-6ea98ba
+            target: dockcross-manylinux_2_28-x64:20250913-6ea98ba
+          # v6.0b02 / main image tags
           - source: docker.io/dockcross/manylinux_2_28-x64:20260203-3dfb3ff
             target: dockcross-manylinux_2_28-x64:20260203-3dfb3ff
           - source: quay.io/pypa/manylinux_2_28_aarch64:2025.08.12-1