Set HOME in apertis-image-builder

Docker is defaulting to setting HOME as `/`. When running the suggested
Docker command in apertis-image-recipes for generating images, this is
resulting in a UML kernel panic as UML is unable to create `~/.uml/`.

Set the HOME to point to a location that we know is backed by tmpfs.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>