Note that GM doesn't support gradient:angle so we
have to use a direction. It's radial, not linear,
but is completely acceptable.

Imagemagick silently introduces artifacts and fails
to composite images when the underlying gradient is
of the form { #AAAAAA --> #BBBBBB }, where R=G=B.

Note that { #AAAAAA --> #BBBBBC } works fine. This
is a real bug but isn't a wallpapers problem.

Add a 'prepare-docker' wrapper script, which simply
builds the tarball using a known Graphicsmagick and
not whichever happens to be on the generating system.