This commit adds an initial Containerfile to build an `image-builder`
container.
Use all the DNF commands together and add a cleaning command to the end
of it. This makes fewer layers and keeps the image size slightly
smaller.
Co-authored-by: Simon de Vlieger <supakeen@redhat.com>
Signed-off-by: Simon de Vlieger <supakeen@redhat.com>
