docker: Enhance Docker manifest and Containerfile for platform-specif… #5

Job	Run time
docker-platforms (litex) / docker-amd64 / build-linux/amd64	2m 51s
docker-platforms (zephyr-cp) / docker-arm64 / build-linux/arm64	5m 30s
docker-platforms (zephyr-cp) / docker-amd64 / build-linux/amd64	6m 14s
docker-platforms (litex) / docker-arm64	0s
docker-platforms (litex) / docker-manifest	0s
docker-platforms (zephyr-cp) / docker-manifest	1s
	14m 36s

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