Enable buildkit for Makefile build target
Enable buildkit for Makefile build target

This is set only if it is not already set.
This should give a little speedup to CI builds.

Signed-off-by: Brian Goff <dc73d0991b982a8303e713ef531a313af77e242a@gmail.com>

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