builder: fix empty reference
builder: fix empty reference

Signed-off-by: CrazyMax <2eb8101acbc78c0d8bc50c0413737c84f1fc3bdc@users.noreply.github.com>

Mokolea/docker,jimjag/docker,Mokolea/docker,Ramzec/docker,trebonian/docker,Ramzec/docker,moby/moby,cpuguy83/docker,mahak/docker,thaJeztah/docker,moul/docker,thaJeztah/docker,konstruktoid/docker-upstream,trebonian/docker,jimjag/docker,cpuguy83/docker,moul/docker,moul/docker,mahak/docker,Ramzec/docker,moby/moby,moby/moby,cpuguy83/docker,Mokolea/docker,konstruktoid/docker-upstream,cpuguy83/docker,trebonian/docker,konstruktoid/docker-upstream,trebonian/docker,nawawi/docker,jimjag/docker,nawawi/docker,thaJeztah/docker,mahak/docker,mahak/docker,Mokolea/docker,moul/docker,Ramzec/docker,moby/moby,nawawi/docker,thaJeztah/docker,jimjag/docker,konstruktoid/docker-upstream,nawawi/docker