Update docker to 0.8.0

This also contains the upstream changes to PKGBUILD.

Sorry, this being my first real contribution, I am a bit puzzled on how the other packages are built. I had to add `arch=('armv7h')` in order for _makepkg_ to work for me (on the ODROID-U3). When using `--ignorearch`, it complained about _gd_ missing as a dependency.

Also, I was not able to test any other than the btrfs storage driver, which works ok so far, although there are a few glitches I cannot explain yet. (AUFS needs a patched kernel and device mapper does not like my eMMC.)

Hoping docker will adopt the ARM architecture soon so we do not need such a lot of build arch patches.