Dockerfile: update to download go from git and utilize volume mount.
Dockerfile: update to download go from git and utilize volume mount.

Change-Id: Ib7d32298f6ee7f74e5c6e0da7e1022ed6a23283a
Reviewed-on: https://go-review.googlesource.com/1310
Reviewed-by: David Crawshaw <a2f0bf29257797dd66800b4fbb9945c3e216c004@golang.org>

MaxCy330/mobile,xing/gomobile,zach-klippenstein/mobile,cainiaocome/mobile,goldcaddy77/mobile,qskycolor/mobile,freeman0211/mobile,freeman0211/mobile,DanielWorld/mobile,swisspol/mobile,samanalysis/mobile,cainiaocome/mobile,ppamorim/mobile,DanielWorld/mobile,tempbottle/mobile,cainiaocome/mobile,freeman0211/mobile,zach-klippenstein/mobile,grategames/mobile,grategames/mobile,goldcaddy77/mobile,b54898533/mobile,samanalysis/mobile,swisspol/mobile,cainiaocome/mobile,steeve/mobile,xing/gomobile,0x90sled/mobile,DanielWorld/mobile,golang/mobile,ichu501/mobile,xing/gomobile,goldcaddy77/mobile,ppamorim/mobile,tempbottle/mobile,0x90sled/mobile,MaxCy330/mobile,0x90sled/mobile,lonphy23t/mobile,lonphy23t/mobile,goldcaddy77/mobile,lonphy23t/mobile,steeve/mobile,steeve/mobile,swisspol/mobile,lonphy23t/mobile,cainiaocome/mobile,samanalysis/mobile,MaxCy330/mobile,samanalysis/mobile,tempbottle/mobile,MaxCy330/mobile,tempbottle/mobile,b54898533/mobile,goldcaddy77/mobile,tempbottle/mobile,zach-klippenstein/mobile,xing/gomobile,qskycolor/mobile,ppamorim/mobile,qskycolor/mobile,0x90sled/mobile,MaxCy330/mobile,grategames/mobile,ppamorim/mobile,ichu501/mobile,b54898533/mobile,xing/gomobile,steeve/mobile,golang/mobile,qskycolor/mobile,ppamorim/mobile,swisspol/mobile,ichu501/mobile,ichu501/mobile,swisspol/mobile,golang/mobile,DanielWorld/mobile,zach-klippenstein/mobile,ichu501/mobile,grategames/mobile,freeman0211/mobile,b54898533/mobile,lonphy23t/mobile,samanalysis/mobile,golang/mobile,b54898533/mobile,0x90sled/mobile