Build rootfs using Alpine (apk-static)

Thanks to Eivind Uggedal (uggedal) for `mkimage-alpine.sh` script,
building our own `rootfs` will remove the dependency on external
base images.
Use our own rootfs

`rootfs` is not built by us, so build container from scratch and
include initial version.
Merge pull request #1 from mini-containers/build-rootfs

Build rootfs and remove external dependency