builder: treat unset keep-storage as 0
builder: treat unset keep-storage as 0

Signed-off-by: Tibor Vass <3f1326dff4d85ad4ec938fb33f64575ace96db26@docker.com>

jvanz/docker,rremer/moby,cpuguy83/docker,KarthikNayak/docker,jwhonce/docker,nawawi/docker,bearstech/moby,SvenDowideit/docker,nawawi/docker,cpuguy83/docker,jameszhan/docker,tmp6154/docker,yosifkit/docker,nalind/docker,gnailuy/docker,nawawi/docker,rremer/moby,jlhawn/docker,jvanz/docker,moby/moby,AkihiroSuda/docker,mahak/docker,dmcgowan/docker,justincormack/docker,justincormack/docker,bearstech/moby,tmp6154/docker,yosifkit/docker,kojiromike/docker,bearstech/moby,yosifkit/docker,bearstech/moby,yongtang/docker,paralin/docker,paralin/docker,laijs/docker,konstruktoid/docker-1,ibuildthecloud/docker,vdemeester/docker,rancher/docker,YuPengZTE/docker,thaJeztah/docker,estesp/docker,cpuguy83/docker,flavio/docker,dmcgowan/docker,konstruktoid/docker-1,gnailuy/docker,unclejack/docker,gnailuy/docker,paralin/docker,jameszhan/docker,kolyshkin/moby,kojiromike/docker,konstruktoid/docker-upstream,Ramzec/docker,laijs/docker,laijs/moby,konstruktoid/docker-upstream,Mokolea/docker,moby/moby,laijs/docker,x1022as/docker,ibuildthecloud/docker,docker/docker,YuPengZTE/docker,jimjag/docker,moul/docker,trebonian/docker,docker/docker,SvenDowideit/docker,kojiromike/docker,justincormack/docker,jwhonce/docker,flavio/docker,jlhawn/docker,moby/moby,flavio/docker,AkihiroSuda/docker,Ramzec/docker,paralin/docker,mahak/docker,jwhonce/docker,crosbymichael/docker,konstruktoid/docker-upstream,KarthikNayak/docker,thaJeztah/docker,dims/docker,unclejack/docker,AkihiroSuda/docker,nalind/docker,ibuildthecloud/docker,cpuguy83/docker,Ramzec/docker,pradipd/moby,AkihiroSuda/docker,yongtang/docker,crosbymichael/docker,vdemeester/docker,dims/docker,vdemeester/docker,moby/moby,crosbymichael/docker,mikebrow/docker,trebonian/docker,jthelin/docker,estesp/docker,laijs/moby,nalind/docker,djs55/docker,dims/docker,jimjag/docker,estesp/docker,pradipd/moby,rancher/docker,jimjag/docker,nawawi/docker,kolyshkin/docker,djs55/docker,SvenDowideit/docker,unclejack/docker,djs55/docker,moul/docker,ibuildthecloud/docker,jvanz/docker,kolyshkin/moby,kolyshkin/moby,Ramzec/docker,mikebrow/docker,trebonian/docker,moul/docker,scotttlin/docker,trebonian/docker,maxamillion/docker,rancher/docker,yongtang/docker,docker/docker,gnailuy/docker,ibuildthecloud/docker,KarthikNayak/docker,Mokolea/docker,konstruktoid/docker-upstream,x1022as/docker,laijs/moby,mahak/docker,mahak/docker,maxamillion/docker,kolyshkin/docker,jameszhan/docker,yosifkit/docker,jimjag/docker,x1022as/docker,thaJeztah/docker,flavio/docker,nalind/docker,tmp6154/docker,mikebrow/docker,Mokolea/docker,maxamillion/docker,dmcgowan/docker,YuPengZTE/docker,jlhawn/docker,moul/docker,jthelin/docker,rremer/moby,scotttlin/docker,Mokolea/docker,pradipd/moby,konstruktoid/docker-1,scotttlin/docker,thaJeztah/docker