support custom path for running in containers

[zhangguanzhang]

$ docker run --rm -ti -v /sys/firmware/:/sys/firmware/ alpine
/ # ls -l /sys/firmware/
total 0
/ # 
$ docker run --rm -ti -v /sys/firmware/:/rootfs/sys/firmware/ alpine
/ # ls -l /rootfs/sys/firmware/
total 0
drwxr-xr-x    5 root     root             0 Nov  6 01:53 acpi
drwxr-xr-x    3 root     root             0 Nov  6 01:53 dmi
drwxr-xr-x   10 root     root             0 Nov  6 01:53 memmap
drwxr-xr-x    4 root     root             0 Nov  6 01:53 qemu_fw_cfg