apt cache for docker build

the main proposal of this research is to enable cached apt archives for docker build.

first apt cache can be saved by a docker volume, but due to `${SDCARD}/var/cache/apt/archives` is not a constant, then we can't use `-v=apt-cache:${SDCARD}/var/cache/apt/archives` to enable it.

so I use `-v=apt-cache:/root/.apt` as a tmp place for apt cache, when need `apt install` in `${SDCARD}`, use `mount -o bind /root/.apt ${SDCARD}/var/cache/apt/archives` and after install finishes, `umount ${SDCARD}/var/cache/apt/archives`

this looks perfect, but after I check /root/.apt, everything is gone.

I double checked whether *.deb are saved to /root/.apt, I can find them before `umount`, or even use apt-cache in another container: `docker run --privileged --rm -v=apt-cache:/test -it ubuntu:18.04 bash` I can find debs in /test folder, and gone after `umount`

I really don't know why.