do not ignore local build-contexts starting with "github.com"
do not ignore local build-contexts starting with "github.com"

Docker special-cases build-contexts starting with `github.com`, and
treats them as remote URLs.

Because of this special treatment, local build contexts in a directory
named "github.com" are ignored by `docker build`.

This patch changes the way the build-context is detected and first
checks if a local path with the given name exists before considering
it to be a remote URL.

Before this change;

    $ mkdir -p github.com/foo/bar && echo -e "FROM scratch\nLABEL iam=local" > github.com/foo/bar/Dockerfile

    $ docker build -t dont-ignore-me github.com/foo/bar
    Username for 'https://github.com':

After this change;

    $ mkdir -p github.com/foo/bar && echo -e "FROM scratch\nLABEL iam=local" > github.com/foo/bar/Dockerfile

    $ docker build -t dont-ignore-me github.com/foo/bar
    Sending build context to Docker daemon 2.048 kB
    Step 1/2 : FROM scratch
     --->
    Step 2/2 : LABEL iam local
     ---> Using cache
     ---> ae2c603fe970
    Successfully built ae2c603fe970

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

coreos/docker,noironetworks/docker,moby/moby,bonczj/moby-json-file-logger,unclejack/docker,ripcurld0/moby,AkihiroSuda/docker,resin-io/docker,mikebrow/docker,miminar/docker,mixja/docker,mkumatag/docker,rhatdan/docker,ndeloof/docker,dperny/docker,miminar/docker,laijs/moby,estesp/docker,madhanrm/docker,kencochrane/docker,Mokolea/docker,mikebrow/docker,lixiaobing10051267/docker,Mokolea/docker,bearstech/moby,darrenstahlmsft/docker,whuwxl/docker,sdurrheimer/docker,HideoYamauchi/docker,tianon/docker,konstruktoid/docker-upstream,ibuildthecloud/docker,dperny/docker,shhsu/docker,sterchelen/docker,vdemeester/docker,srust/docker,nalind/docker,mtesselH/docker,hqhq/moby,SvenDowideit/docker,mahak/docker,qudongfang/docker,yongtang/docker,charleszheng44/moby,sarnautov/moby,qudongfang/docker,mike1729/docker,gnailuy/docker,tonistiigi/docker,moby/moby,shishir-a412ed/docker,konstruktoid/docker-upstream,hqhq/moby,JMesser81/docker,jthelin/docker,dccurtis/docker,moul/docker,Syntaxide/docker,darrenstahlmsft/docker,KingEmet/docker,jimjag/docker,johnstep/moby-moby,PatrickLang/moby,naveed-jamil-tenpearls/moby,estesp/docker,prertik/docker,mikebrow/docker,paralin/docker,dsheets/moby,lixiaobing10051267/docker,trebonian/docker,crosbymichael/docker,rancher/docker,nawawi/docker,naveed-jamil-tenpearls/moby,dgageot/docker,sdurrheimer/docker,Ramzec/docker,xuxinkun/docker,bearstech/moby,ColinHebert/docker,POLJM242/docker,dccurtis/docker,tonistiigi/docker,naveed-jamil-tenpearls/moby,jwhonce/docker,jthelin/docker,imkin/moby,catinthesky/docker,kolyshkin/moby,alfred-landrum/docker,erikh/docker,gnailuy/docker,ibuildthecloud/docker,raja-sami-10p/moby,Syntaxide/docker,sarnautov/moby,trebonian/docker,x1022as/docker,flavio/docker,mixja/docker,gjaskiewicz/moby,erikstmartin/docker,whuwxl/docker,maxamillion/docker,jameszhan/docker,prertik/docker,mahak/docker,sarnautov/moby,pradipd/moby,tophj-ibm/moby,madhanrm/docker,mkumatag/docker,thaJeztah/docker,gschaetz/moby,tianon/docker,liaoqingwei/docker,raja-sami-10p/moby,jameszhan/docker,jlhawn/docker,SvenDowideit/docker,miminar/docker,jvanz/docker,ffoysal/docker,trebonian/docker,resin-io/docker,crosbymichael/docker,docker/docker,yosifkit/docker,ColinHebert/docker,tmp6154/docker,qudongfang/docker,catinthesky/docker,tonistiigi/docker,Ramzec/docker,gschaetz/moby,pradipd/moby,unclejack/docker,ripcurld0/moby,coolljt0725/docker,laijs/moby,coolljt0725/docker,paralin/docker,ripcurld0/moby,nawawi/docker,ffoysal/docker,darrenstahlmsft/docker,amatsus/docker,pramodhkp/moby,kolyshkin/moby,kojiromike/docker,mahak/docker,liaoqingwei/docker,bearstech/moby,kolyshkin/docker,q384566678/docker,johnstep/moby-moby,mahak/docker,erikstmartin/docker,cyphar/docker,yongtang/docker,fcrisciani/docker,tophj-ibm/moby,duglin/docker,mtesselH/docker,KarthikNayak/docker,mstanleyjones/docker,noironetworks/docker,imkin/moby,yummypeng/docker,moul/docker,kojiromike/docker,jlhawn/docker,moby/moby,gjaskiewicz/moby,yongtang/docker,jwhonce/docker,charleszheng44/moby,danyalKhaliq10p/moby,YuPengZTE/docker,coreos/docker,AkihiroSuda/docker,sterchelen/docker,vdemeester/docker,Ramzec/docker,PatrickLang/moby,raja-sami-10p/moby,gnailuy/docker,paralin/docker,gschaetz/moby,mike1729/docker,allencloud/docker,estesp/docker,djs55/docker,SvenDowideit/docker,lixiaobing1/docker,shhsu/docker,tophj-ibm/moby,HideoYamauchi/docker,nalind/docker,akira-baruah/docker,konstruktoid/docker-1,paralin/docker,kencochrane/docker,docker/docker,srust/docker,crosbymichael/docker,ibuildthecloud/docker,ibuildthecloud/docker,darrenstahlmsft/moby,darrenstahlmsft/moby,docker/docker,ffoysal/docker,unclejack/docker,KarthikNayak/docker,x1022as/docker,diogomonica/docker,q384566678/docker,cpuguy83/docker,tmp6154/docker,darrenstahlmsft/moby,konstruktoid/docker-1,fcrisciani/docker,dgageot/docker,dsheets/moby,jvanz/docker,yosifkit/docker,tophj-ibm/docker,rremer/moby,mixja/docker,jlhawn/docker,yummypeng/docker,erikh/docker,YuPengZTE/docker,nawawi/docker,justincormack/docker,hqhq/moby,jimjag/docker,Mokolea/docker,dmcgowan/docker,johnstep/moby-moby,duglin/docker,nawawi/docker,rhatdan/docker,dperny/docker,coolljt0725/docker,rancher/docker,bonczj/moby-json-file-logger,yasiramin10p/moby,thaJeztah/docker,akira-baruah/docker,thtanaka/docker,whuwxl/docker,gnailuy/docker,cpuguy83/docker,moul/docker,dims/docker,YuPengZTE/docker,diogomonica/docker,vdemeester/docker,bonczj/moby-json-file-logger,nalind/docker,nalind/docker,diogomonica/docker,jimjag/docker,shhsu/docker,laijs/moby,mkumatag/docker,lixiaobing10051267/docker,rremer/moby,prertik/docker,ndeloof/docker,danyalKhaliq10p/moby,djs55/docker,tklauser/docker,dims/docker,dmcgowan/docker,bearstech/moby,thtanaka/docker,ColinHebert/docker,sterchelen/docker,haveatry/docker,charleszheng44/moby,shishir-a412ed/docker,tophj-ibm/docker,tmp6154/docker,KingEmet/docker,dccurtis/docker,tklauser/docker,mtesselH/docker,alfred-landrum/docker,duglin/docker,yosifkit/docker,flavio/docker,lixiaobing1/docker,laijs/docker,srust/docker,flavio/docker,tklauser/docker,ibuildthecloud/docker,laijs/docker,yosifkit/docker,mstanleyjones/docker,dims/docker,tophj-ibm/docker,POLJM242/docker,michael-holzheu/docker,PatrickLang/moby,noironetworks/docker,konstruktoid/docker-upstream,ndeloof/docker,dsheets/moby,gjaskiewicz/moby,HideoYamauchi/docker,cpuguy83/docker,mike1729/docker,erikh/docker,x1022as/docker,flavio/docker,kolyshkin/docker,moypray/docker,michael-holzheu/docker,sdurrheimer/docker,pradipd/moby,cyphar/docker,amatsus/docker,justincormack/docker,dgageot/docker,djs55/docker,yasiramin10p/moby,yasiramin10p/moby,thtanaka/docker,thaJeztah/docker,cyphar/docker,maxamillion/docker,haveatry/docker,Ramzec/docker,kolyshkin/moby,justincormack/docker,amatsus/docker,Mokolea/docker,KingEmet/docker,trebonian/docker,resin-io/docker,liaoqingwei/docker,yummypeng/docker,xuxinkun/docker,allencloud/docker,moypray/docker,pramodhkp/moby,xuxinkun/docker,kojiromike/docker,AkihiroSuda/docker,konstruktoid/docker-1,rhatdan/docker,lixiaobing1/docker,moby/moby,scotttlin/docker,rancher/docker,laijs/docker,jvanz/docker,Syntaxide/docker,pramodhkp/moby,coreos/docker,haveatry/docker,madhanrm/docker,dmcgowan/docker,q384566678/docker,rremer/moby,moul/docker,imkin/moby,michael-holzheu/docker,jwhonce/docker,JMesser81/docker,shishir-a412ed/docker,cpuguy83/docker,mstanleyjones/docker,scotttlin/docker,alfred-landrum/docker,kencochrane/docker,fcrisciani/docker,moypray/docker,danyalKhaliq10p/moby,JMesser81/docker,jimjag/docker,allencloud/docker,scotttlin/docker,AkihiroSuda/docker,erikstmartin/docker,KarthikNayak/docker,thaJeztah/docker,jameszhan/docker,catinthesky/docker,konstruktoid/docker-upstream,maxamillion/docker