Change demo server to a container-based setup

The FreedomBox demo server running at https://demo.freedombox.org is currently an AWS EC2 instance which makes it available only on AWS. Running this as a container on a VPS instead has several advantages:

• more control over the setup process
• independence from a GAFAM cloud provider
• choice of running this on our own hardware or a VPS from a small hosting provider
• VPSes usually cost less than EC2 instances

Tasks

• Install package systemd-container
• Pull a customized amd64 build for the container
• container stop, container destroy and container up
• port-forwarding settings
• bind mounts (certificate)
• systemd-timer service to run the script