Update Docker file to Go 1.8
Update Docker file to Go 1.8

* new go-version, switch to alpine (reduce size by 500mb)
* use govendor

mdhender/hugo,mdhender/hugo,mitchchn/hugo,digitalcraftsman/hugo,digitalcraftsman/hugo,anthonyfok/hugo,mitchchn/hugo,anthonyfok/hugo,anthonyfok/hugo,fedelibre/hugo,mdhender/hugo,gohugoio/hugo,fedelibre/hugo,gohugoio/hugo,fedelibre/hugo,fedelibre/hugo,gohugoio/hugo,mdhender/hugo