provisioners/docker: Only assign the `image` config in case it was not specified
provisioners/docker: Only assign the `image` config in case it was not specified

tschortsch/vagrant,genome21/vagrant,modulexcite/vagrant,otagi/vagrant,carlosefr/vagrant,tbriggs-curse/vagrant,tbarrongh/vagrant,benizi/vagrant,bryson/vagrant,dhoer/vagrant,muhanadra/vagrant,signed8bit/vagrant,BlakeMesdag/vagrant,dustymabe/vagrant,p0deje/vagrant,apertoso/vagrant,Chhunlong/vagrant,legal90/vagrant,Ninir/vagrant,Sgoettschkes/vagrant,mpoeter/vagrant,darkn3rd/vagrant,philoserf/vagrant,shtouff/vagrant,muhanadra/vagrant,tbriggs-curse/vagrant,gajdaw/vagrant,jean/vagrant,aaam/vagrant,marxarelli/vagrant,jhoblitt/vagrant,taliesins/vagrant,invernizzi-at-google/vagrant,invernizzi-at-google/vagrant,aneeshusa/vagrant,tbriggs-curse/vagrant,blueyed/vagrant,patrys/vagrant,teotihuacanada/vagrant,Endika/vagrant,philoserf/vagrant,gbarberi/vagrant,sideci-sample/sideci-sample-vagrant,wangfakang/vagrant,sferik/vagrant,denisbr/vagrant,jmanero/vagrant,tbarrongh/vagrant,fnewberg/vagrant,ianmiell/vagrant,nickryand/vagrant,pwnall/vagrant,marxarelli/vagrant,mwarren/vagrant,sideci-sample/sideci-sample-vagrant,mitchellh/vagrant,janek-warchol/vagrant,stephancom/vagrant,Endika/vagrant,lukebakken/vagrant,ianmiell/vagrant,bdwyertech/vagrant,tjanez/vagrant,modulexcite/vagrant,mkuzmin/vagrant,vamegh/vagrant,bdwyertech/vagrant,gpkfr/vagrant,johntron/vagrant,crashlytics/vagrant,juiceinc/vagrant,doy/vagrant,bheuvel/vagrant,PatrickLang/vagrant,sferik/vagrant,jhoblitt/vagrant,philoserf/vagrant,ferventcoder/vagrant,iNecas/vagrant,sax/vagrant,shtouff/vagrant,lonniev/vagrant,fnewberg/vagrant,Avira/vagrant,evverx/vagrant,bryson/vagrant,teotihuacanada/vagrant,TheBigBear/vagrant,myrjola/vagrant,carlosefr/vagrant,clinstid/vagrant,apertoso/vagrant,PatrickLang/vagrant,jean/vagrant,mwrock/vagrant,mwrock/vagrant,gbarberi/vagrant,benh57/vagrant,aaam/vagrant,PatOShea/vagrant,kalabiyau/vagrant,loren-osborn/vagrant,gbarberi/vagrant,jkburges/vagrant,jkburges/vagrant,miguel250/vagrant,kamigerami/vagrant,krig/vagrant,bshurts/vagrant,myrjola/vagrant,justincampbell/vagrant,legal90/vagrant,petems/vagrant,genome21/vagrant,zsjohny/vagrant,denisbr/vagrant,teotihuacanada/vagrant,signed8bit/vagrant,Sgoettschkes/vagrant,muhanadra/vagrant,tknerr/vagrant,ArloL/vagrant,sideci-sample/sideci-sample-vagrant,BlakeMesdag/vagrant,lonniev/vagrant,rivy/vagrant,fnewberg/vagrant,kamazee/vagrant,p0deje/vagrant,dustymabe/vagrant,Chhed13/vagrant,denisbr/vagrant,wkolean/vagrant,TheBigBear/vagrant,MiLk/vagrant,vamegh/vagrant,shtouff/vagrant,invernizzi-at-google/vagrant,zsjohny/vagrant,Chhed13/vagrant,benh57/vagrant,jberends/vagrant,rivy/vagrant,lonniev/vagrant,myrjola/vagrant,jmanero/vagrant,signed8bit/vagrant,channui/vagrant,Avira/vagrant,darkn3rd/vagrant,TheBigBear/vagrant,jkburges/vagrant,dustymabe/vagrant,mitchellh/vagrant,philoserf/vagrant,Avira/vagrant,aneeshusa/vagrant,jberends/vagrant,theist/vagrant,ferventcoder/vagrant,jtopper/vagrant,tknerr/vagrant,johntron/vagrant,dustymabe/vagrant,iNecas/vagrant,Endika/vagrant,mwarren/vagrant,doy/vagrant,aaam/vagrant,ianmiell/vagrant,tjanez/vagrant,juiceinc/vagrant,loren-osborn/vagrant,mkuzmin/vagrant,loren-osborn/vagrant,kamazee/vagrant,myrjola/vagrant,nickryand/vagrant,carlosefr/vagrant,evverx/vagrant,tjanez/vagrant,evverx/vagrant,rivy/vagrant,muhanadra/vagrant,krig/vagrant,Sgoettschkes/vagrant,philwrenn/vagrant,petems/vagrant,ArloL/vagrant,dhoer/vagrant,vamegh/vagrant,obnoxxx/vagrant,petems/vagrant,benizi/vagrant,bheuvel/vagrant,sni/vagrant,cgvarela/vagrant,sferik/vagrant,juiceinc/vagrant,gitebra/vagrant,darkn3rd/vagrant,TheBigBear/vagrant,MiLk/vagrant,h4ck3rm1k3/vagrant,sni/vagrant,juiceinc/vagrant,tomfanning/vagrant,apertoso/vagrant,mwarren/vagrant,mwrock/vagrant,kalabiyau/vagrant,senglin/vagrant,tschortsch/vagrant,otagi/vagrant,crashlytics/vagrant,bryson/vagrant,genome21/vagrant,krig/vagrant,bryson/vagrant,taliesins/vagrant,miguel250/vagrant,lukebakken/vagrant,aaam/vagrant,miguel250/vagrant,legal90/vagrant,dharmab/vagrant,tbriggs-curse/vagrant,jhoblitt/vagrant,legal90/vagrant,vamegh/vagrant,crashlytics/vagrant,chrisvire/vagrant,jean/vagrant,theist/vagrant,h4ck3rm1k3/vagrant,obnoxxx/vagrant,tbarrongh/vagrant,patrys/vagrant,aneeshusa/vagrant,sax/vagrant,cgvarela/vagrant,BlakeMesdag/vagrant,otagi/vagrant,bheuvel/vagrant,marxarelli/vagrant,philwrenn/vagrant,jtopper/vagrant,doy/vagrant,gpkfr/vagrant,benizi/vagrant,shtouff/vagrant,crashlytics/vagrant,iNecas/vagrant,jmanero/vagrant,gitebra/vagrant,gitebra/vagrant,gpkfr/vagrant,justincampbell/vagrant,obnoxxx/vagrant,Sgoettschkes/vagrant,patrys/vagrant,miguel250/vagrant,kamazee/vagrant,kamigerami/vagrant,otagi/vagrant,bdwyertech/vagrant,fnewberg/vagrant,dharmab/vagrant,Ninir/vagrant,blueyed/vagrant,nickryand/vagrant,genome21/vagrant,teotihuacanada/vagrant,philwrenn/vagrant,benizi/vagrant,modulexcite/vagrant,dharmab/vagrant,jfchevrette/vagrant,bdwyertech/vagrant,jhoblitt/vagrant,chrisvire/vagrant,gajdaw/vagrant,stephancom/vagrant,stephancom/vagrant,dhoer/vagrant,wkolean/vagrant,jfchevrette/vagrant,petems/vagrant,gpkfr/vagrant,mkuzmin/vagrant,clinstid/vagrant,clinstid/vagrant,tschortsch/vagrant,janek-warchol/vagrant,kamigerami/vagrant,mwrock/vagrant,jberends/vagrant,channui/vagrant,Ninir/vagrant,taliesins/vagrant,p0deje/vagrant,ianmiell/vagrant,lukebakken/vagrant,ferventcoder/vagrant,justincampbell/vagrant,bshurts/vagrant,pwnall/vagrant,cgvarela/vagrant,stephancom/vagrant,MiLk/vagrant,wkolean/vagrant,PatOShea/vagrant,krig/vagrant,janek-warchol/vagrant,tjanez/vagrant,sni/vagrant,aneeshusa/vagrant,h4ck3rm1k3/vagrant,ArloL/vagrant,h4ck3rm1k3/vagrant,apertoso/vagrant,sax/vagrant,tomfanning/vagrant,Avira/vagrant,Endika/vagrant,rivy/vagrant,johntron/vagrant,lukebakken/vagrant,tbarrongh/vagrant,ferventcoder/vagrant,mpoeter/vagrant,Chhunlong/vagrant,tschortsch/vagrant,tomfanning/vagrant,PatOShea/vagrant,gbarberi/vagrant,signed8bit/vagrant,pwnall/vagrant,wangfakang/vagrant,mpoeter/vagrant,sni/vagrant,senglin/vagrant,tknerr/vagrant,johntron/vagrant,PatOShea/vagrant,tomfanning/vagrant,Chhed13/vagrant,wangfakang/vagrant,kalabiyau/vagrant,denisbr/vagrant,gitebra/vagrant,kalabiyau/vagrant,PatrickLang/vagrant,bshurts/vagrant,lonniev/vagrant,invernizzi-at-google/vagrant,dhoer/vagrant,wangfakang/vagrant,tknerr/vagrant,jtopper/vagrant,senglin/vagrant,kamazee/vagrant,mwarren/vagrant,darkn3rd/vagrant,bheuvel/vagrant,doy/vagrant,modulexcite/vagrant,chrisroberts/vagrant,sax/vagrant,Chhunlong/vagrant,jberends/vagrant,bshurts/vagrant,chrisroberts/vagrant,ArloL/vagrant,kamigerami/vagrant,Chhed13/vagrant,jfchevrette/vagrant,Chhunlong/vagrant,blueyed/vagrant,chrisroberts/vagrant,jmanero/vagrant,blueyed/vagrant,chrisvire/vagrant,marxarelli/vagrant,janek-warchol/vagrant,jkburges/vagrant,channui/vagrant,zsjohny/vagrant,mpoeter/vagrant,cgvarela/vagrant,chrisroberts/vagrant,mkuzmin/vagrant,mitchellh/vagrant,wkolean/vagrant,patrys/vagrant,senglin/vagrant,gajdaw/vagrant,jean/vagrant,jtopper/vagrant,carlosefr/vagrant,zsjohny/vagrant,mitchellh/vagrant,justincampbell/vagrant,dharmab/vagrant,chrisvire/vagrant,philwrenn/vagrant,taliesins/vagrant,nickryand/vagrant,benh57/vagrant,jfchevrette/vagrant,benh57/vagrant,theist/vagrant,pwnall/vagrant,PatrickLang/vagrant,loren-osborn/vagrant,theist/vagrant