go.mobile: copy the local version of go.mobile instead of downloading from the net.
go.mobile: copy the local version of go.mobile instead of downloading from the net.

The purpose of this Dockerfile is to make it easier to hack on go.mobile.
'go get' is still desirable for preinstall necessary dependencies.

LGTM=crawshaw, adg
R=adg, crawshaw
CC=golang-codereviews
https://golang.org/cl/155480043

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