functional tests: use different image names for docker vol semantics test
functional tests: use different image names for docker vol semantics test

We were only testing the last generated image, and the other cases
worked because there was no volume being mounted there and hence the
files were visible.

Name each test image differently so we actually do the tests.

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