docker: Update build argument for Docker build process

Changed the build argument from `cpy_platform` to `BUILD_PLATFORM` for clarity and consistency in the Docker build configuration.

Signed-off-by: Chiho Sin <[email protected]>

Author: chihosin

.github/workflows/docker_build.yml

@@ -80,4 +80,4 @@ jobs:
           labels: ${{ steps.meta.outputs.labels }}
           platforms: ${{ inputs.platform }}
           build-args: |
-            cpy_platform=${{ inputs.cpy_platform }}
+            BUILD_PLATFORM=${{ inputs.cpy_platform }}