Allow installation with lxc-docker
Allow installation with lxc-docker

This will allow developers to run an `upgrade` on existing installations without breaking because docker-engine isn't yet in their apt lists...

[ci skip]
jimjag/dokku,jazzzz/dokku,bobvanderlinden/dokku,progrium/dokku,jazzzz/dokku,sseemayer/dokku,elia/dokku,progrium/dokku,expa/dokku,sseemayer/dokku,bobvanderlinden/dokku,u2mejc/dokku,Evlos/dokku,jazzzz/dokku,bobvanderlinden/dokku,johnfraney/dokku,alessio/dokku,Evlos/dokku,fruitl00p/dokku,Cactusbone/dokku,amaltson/dokku,alexquick/dokku,fmoliveira/dokku,netbrick/dokku,matto1990/dokku,callahad/dokku,expa/dokku,matto1990/dokku,amaltson/dokku,callahad/dokku,eljojo/dokku,codeconfab/dokku,pmvieira/dokku,progrium/dokku,claytonbrown/dokku,Cactusbone/dokku,netbrick/dokku,Cactusbone/dokku,alessio/dokku,callahad/dokku,netbrick/dokku,alessio/dokku,johnfraney/dokku,bobvanderlinden/dokku,matto1990/dokku,sseemayer/dokku,Evlos/dokku,amaltson/dokku,crisward/dokku,Cactusbone/dokku,elia/dokku,expa/dokku,fruitl00p/dokku,AnselZhangGit/dokku,jazzzz/dokku,eljojo/dokku,jimjag/dokku,claytonbrown/dokku,bobvanderlinden/dokku,progrium/dokku,u2mejc/dokku,alessio/dokku,matto1990/dokku,callahad/dokku,amaltson/dokku,progrium/dokku,alexquick/dokku,alexquick/dokku,jazzzz/dokku,callahad/dokku,johnfraney/dokku,codeconfab/dokku,pmvieira/dokku,progrium/dokku,Evlos/dokku,Cactusbone/dokku,jazzzz/dokku,claytonbrown/dokku,dokku/dokku,elia/dokku,sseemayer/dokku,expa/dokku,progrium/dokku,alexquick/dokku,fruitl00p/dokku,codeconfab/dokku,elia/dokku,jazzzz/dokku,eljojo/dokku,alessio/dokku,alexquick/dokku,econya/dokku,alexquick/dokku,netbrick/dokku,jimjag/dokku,sseemayer/dokku,Evlos/dokku,fmoliveira/dokku,expa/dokku,vijayviji/dokku,AnselZhangGit/dokku,alessio/dokku,crisward/dokku,vijayviji/dokku,AnselZhangGit/dokku,Evlos/dokku,fruitl00p/dokku,Evlos/dokku,econya/dokku,expa/dokku,pmvieira/dokku,vijayviji/dokku,johnfraney/dokku,vijayviji/dokku,AnselZhangGit/dokku,codeconfab/dokku,fruitl00p/dokku,elia/dokku,econya/dokku,u2mejc/dokku,expa/dokku,u2mejc/dokku,netbrick/dokku,AnselZhangGit/dokku,johnfraney/dokku,codeconfab/dokku,matto1990/dokku,claytonbrown/dokku,crisward/dokku,u2mejc/dokku,fruitl00p/dokku,codeconfab/dokku,vijayviji/dokku,Cactusbone/dokku,bobvanderlinden/dokku,vijayviji/dokku,u2mejc/dokku,fmoliveira/dokku,claytonbrown/dokku,econya/dokku,Evlos/dokku,johnfraney/dokku,elia/dokku,claytonbrown/dokku,jimjag/dokku,jimjag/dokku,codeconfab/dokku,sseemayer/dokku,Cactusbone/dokku,alexquick/dokku,sseemayer/dokku,matto1990/dokku,johnfraney/dokku,AnselZhangGit/dokku,claytonbrown/dokku,matto1990/dokku,sseemayer/dokku,vijayviji/dokku,eljojo/dokku,elia/dokku,dokku/dokku,u2mejc/dokku,jimjag/dokku,claytonbrown/dokku,econya/dokku,progrium/dokku,eljojo/dokku,fmoliveira/dokku,codeconfab/dokku,dokku/dokku,pmvieira/dokku,bobvanderlinden/dokku,dokku/dokku,econya/dokku,fmoliveira/dokku,dokku/dokku,bobvanderlinden/dokku,netbrick/dokku,pmvieira/dokku,dokku/dokku,callahad/dokku,jimjag/dokku,dokku/dokku,econya/dokku,fruitl00p/dokku,eljojo/dokku,jimjag/dokku,AnselZhangGit/dokku,fmoliveira/dokku,netbrick/dokku,netbrick/dokku,expa/dokku,johnfraney/dokku,dokku/dokku,AnselZhangGit/dokku,fmoliveira/dokku,crisward/dokku,vijayviji/dokku,u2mejc/dokku,alessio/dokku,amaltson/dokku,matto1990/dokku,pmvieira/dokku,Cactusbone/dokku,callahad/dokku,pmvieira/dokku,amaltson/dokku,amaltson/dokku,crisward/dokku,alexquick/dokku,amaltson/dokku,jazzzz/dokku,eljojo/dokku,callahad/dokku,econya/dokku,crisward/dokku,eljojo/dokku,fruitl00p/dokku,elia/dokku,pmvieira/dokku,crisward/dokku,crisward/dokku,alessio/dokku,fmoliveira/dokku