Only set DOCKER_CROSSPLATFORMS from env if it's set
Only set DOCKER_CROSSPLATFORMS from env if it's set

This allows for the default to come from the image/`Dockerfile` rather
than being unconditionally overwritten by the environment.

Signed-off-by: Andrew "Tianon" Page <4b2b69742da531f1900c4c4748d7a0ecda59626a@gmail.com>

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