rkt/image: render images only if we're root
rkt/image: render images only if we're root

To render images, we need access to the tree store directories and
capabilities to do a chroot.

We could allow the rkt group to access the tree store directories and
not do a chroot when rendering but since this rendering-on-fetch is just
an optimization, it's simpler to just skip the rendering if we fetch as
non-root.

achanda/rkt,rkt/rkt,dgonyeo/rkt,yifan-gu/rocket,mike1729/rkt,Wilybald/rkt,monstermunchkin/rkt,alban/rkt,achanda/rkt,joshix/rkt,dgonyeo/rkt,akshaykarle/rkt,sstabellini/rkt,joshix/rkt,sstabellini/rkt,joshix/rkt,shobhit85/rkt,cab105/rkt,jonboulle/rkt,kinvolk/rkt,lucasallan/rkt,squeed/rkt,kinvolk/rkt,mike1729/rkt,jakon89/rkt,euank/rkt,ybubnov/rkt,markllama/rkt,lucasallan/rkt,tmrts/rkt,Wilybald/rkt,akshaykarle/rkt,s-urbaniak/rkt,sgotti/rkt,dgonyeo/rkt,Wilybald/rkt,rkt/rkt,yifan-gu/rocket,rkt/rkt,yifan-gu/rkt,euank/rkt,squeed/rkt,euank/rkt,lucasallan/rkt,jjlakis/rkt,Shashwatsh/rkt,yifan-gu/rocket,lucasallan/rkt,tmrts/rkt,jakon89/rkt,yifan-gu/rkt,akshaykarle/rkt,nhlfr/rkt,rkt/rkt,coreos/rkt,jonboulle/rkt,matthaias/rkt,kinvolk/rkt,nhlfr/rkt,alban/rkt,kinvolk/rkt,ybubnov/rkt,monstermunchkin/rkt,sgotti/rkt,achanda/rkt,sstabellini/rkt,squeed/rkt,mike1729/rkt,markllama/rkt,nhlfr/rkt,Wilybald/rkt,s-urbaniak/rkt,yifan-gu/rkt,Shashwatsh/rkt,Shashwatsh/rkt,intelsdi-x/rkt,s-urbaniak/rkt,cab105/rkt,ybubnov/rkt,intelsdi-x/rkt,cab105/rkt,shobhit85/rkt,matthaias/rkt,shobhit85/rkt,jakon89/rkt,tmrts/rkt,jjlakis/rkt,coreos/rkt,joshix/rkt,matthaias/rkt,markllama/rkt,sgotti/rkt,alban/rkt,jjlakis/rkt,monstermunchkin/rkt,coreos/rkt,intelsdi-x/rkt,jonboulle/rkt