timedatectl fails with 'bus unavailable' or so.

The downloads allows to list available images and to download, with a
progress bar 🎉 !

The docker package was some dock for KDE4 or something.
There is no better way to get a recent docker version for raspbian than
to use the convenience script, as stated in the documentation: https://docs.docker.com/install/linux/docker-ce/debian/#install-using-the-convenience-script

A full workflow would look like this:

```sh
./prepare_image.sh && ./mount.sh && sudo ./copy.sh $HOME/.ssh/id_rsa.pub && cp ./setup.sh root/tmp/ && ./chroot.sh && ./umount.sh
```

- prepare_image.sh: loads a fresh raspbian-lite, extracts it and expands
the root partition to ~3.7GB
- mount.sh: mounts the image in rootfs/
- copy.sh: copies some files into the root filesystem, e.g
wpa_supplicant.conf, authorizes SSH key
- setup.sh: updates and installs packages (must be called manually in
chroot)
- chroot.sh: opens a qemu-arm based chroot
- umount.sh: as the name says