Add aarch64 docker image

This is the first step, next is to push aarch64 images for all the images we currently build (we need using docker manifests for that)

Fixes #4 (closed)

artifact-cache/prepare.sh

@@ -2,9 +2,8 @@
 
 set -eu
 
-dnf install -y gcc redhat-rpm-config
-dnf install -y python3-devel
 dnf install -y nginx openssh-server ostree python3-gobject python3-psutil
+dnf install -y python3-ruamel-yaml
 
 # Git is needed so `pip3 install .` can detect the version number of
 # the BuildStream package. We remove it in the cleanup.sh script as