Allow our robots to build a Debian sid image

Add similar stuff as we have in our main tree (vagrant/provision/assets/build-tails, auto/build and alike) so that a simple robot knows how to build the image.

Parent Task: #6349 (closed)

Original created by @sajolida on 6351 (Redmine)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information