Dockerfile: update to latest syntax
Dockerfile: update to latest syntax

It was pinned to the 1.3 version; removing the minor version to
make sure we're on the latest 1.x stable.

Signed-off-by: Sebastiaan van Stijn <64b2b6d12bfe4baae7dad3d018f8cbf6b0e7a044@gone.nl>

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