logspout: Sync build script with upstream

extensions/logspout/build.sh

 #!/bin/sh
 
-# unmodified from:
-# https://github.com/gliderlabs/logspout/blob/67ee3831cbd0594361bb3381380c65bdbeb3c20f/custom/build.sh
+# source: https://github.com/gliderlabs/logspout/blob/621524e/custom/build.sh
 
 set -e
-apk add --update go git mercurial build-base
-mkdir -p /go/src/github.com/gliderlabs
-cp -r /src /go/src/github.com/gliderlabs/logspout
-cd /go/src/github.com/gliderlabs/logspout
-export GOPATH=/go
-go get
+apk add --update go build-base git mercurial ca-certificates
+cd /src
 go build -ldflags "-X main.Version=$1" -o /bin/logspout
 apk del go git mercurial build-base
-rm -rf /go /var/cache/apk/* /root/.glide
+rm -rf /root/go /var/cache/apk/*
 
 # backwards compatibility
 ln -fs /tmp/docker.sock /var/run/docker.sock