Dockerfile.buildx: bump to Go 1.13.10
Dockerfile.buildx: bump to Go 1.13.10

Looks like this Dockerfile was not yet updated; possibly because
`go mod` issues when building buildx 0.3.1.

Given that the upstream repository uses vendoring, I:

- removed the `go mod download` step
- added `-mod=vendor` to use vendoring mode

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

AkihiroSuda/docker,paralin/docker,flavio/docker,ibuildthecloud/docker,yosifkit/docker,trebonian/docker,moul/docker,Ramzec/docker,Mokolea/docker,nawawi/docker,cpuguy83/docker,jimjag/docker,nawawi/docker,bearstech/moby,thaJeztah/docker,mahak/docker,jthelin/docker,moby/moby,flavio/docker,kojiromike/docker,paralin/docker,jvanz/docker,konstruktoid/docker-upstream,mahak/docker,vdemeester/docker,mahak/docker,trebonian/docker,docker/docker,bearstech/moby,Mokolea/docker,Mokolea/docker,moul/docker,cpuguy83/docker,moul/docker,ibuildthecloud/docker,ibuildthecloud/docker,estesp/docker,nalind/docker,thaJeztah/docker,jameszhan/docker,moby/moby,justincormack/docker,cpuguy83/docker,dims/docker,Mokolea/docker,Ramzec/docker,ibuildthecloud/docker,konstruktoid/docker-1,cpuguy83/docker,gnailuy/docker,gnailuy/docker,yosifkit/docker,vdemeester/docker,nalind/docker,bearstech/moby,moul/docker,jimjag/docker,jameszhan/docker,Ramzec/docker,estesp/docker,kojiromike/docker,flavio/docker,konstruktoid/docker-upstream,ibuildthecloud/docker,jthelin/docker,djs55/docker,paralin/docker,djs55/docker,vdemeester/docker,konstruktoid/docker-1,jimjag/docker,flavio/docker,Ramzec/docker,trebonian/docker,thaJeztah/docker,djs55/docker,jvanz/docker,gnailuy/docker,justincormack/docker,moby/moby,estesp/docker,konstruktoid/docker-upstream,nalind/docker,dims/docker,nalind/docker,trebonian/docker,paralin/docker,jameszhan/docker,jimjag/docker,yosifkit/docker,moby/moby,nawawi/docker,jvanz/docker,AkihiroSuda/docker,justincormack/docker,AkihiroSuda/docker,docker/docker,yosifkit/docker,nawawi/docker,dims/docker,mahak/docker,docker/docker,konstruktoid/docker-upstream,kojiromike/docker,gnailuy/docker,bearstech/moby,thaJeztah/docker,konstruktoid/docker-1,AkihiroSuda/docker