install `gitlab-workhorse` at `/usr/local` prefix
install `gitlab-workhorse` at `/usr/local` prefix

Fixes #579

evenh/docker-gitlab,nicolasgnr/docker-gitlab,jrjang/docker-gitlab,ldgeo/docker-gitlab,ldgeo/docker-gitlab,ldgeo/docker-gitlab,vla/docker-gitlab,revayd/docker-gitlab,BirgerK/docker-gitlab,revayd/docker-gitlab,M0E-lnx/gitlab-32bit,maikelvl/docker-gitlab,evenh/docker-gitlab,sameersbn/docker-gitlab,goodrain-apps/docker-gitlab,crobays/docker-gitlab,maikelvl/docker-gitlab,solidnerd/docker-gitlab,3kami3/docker-gitlab,jrjang/docker-gitlab,creativ/docker-gitlab-ee,BirgerK/docker-gitlab,crobays/docker-gitlab,goodrain-apps/docker-gitlab,solidnerd/docker-gitlab,patope/docker-gitlab,deruhu/docker_gitlab,patope/docker-gitlab,iamchanghyunpark/docker-gitlab,vla/docker-gitlab,3kami3/docker-gitlab,M0E-lnx/gitlab-32bit,sameersbn/docker-gitlab,deruhu/docker_gitlab,nicolasgnr/docker-gitlab,iamchanghyunpark/docker-gitlab,creativ/docker-gitlab-ee