Add Dockerfile reference docs for using ARG in FROM
Add Dockerfile reference docs for using ARG in FROM

Also fixed some examples of using `docker build` to clarify that the
positional argument is a directory, not a file.

Also fixed some terminology. Dockerfiles contain instructions, not directives or
commands.

Signed-off-by: Daniel Nephin <6347c07ae509164cffebfb1e2a0d6ed64958db19@docker.com>

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