fix release script gpg key id

Signed-off-by: Jessica Frazelle <acidburn@docker.com>

hack/release.sh

@@ -42,6 +42,7 @@ EOF
 [ "$AWS_ACCESS_KEY" ] || usage
 [ "$AWS_SECRET_KEY" ] || usage
 [ "$GPG_PASSPHRASE" ] || usage
+: ${GPG_KEYID:=releasedocker}
 [ -d /go/src/github.com/docker/docker ] || usage
 cd /go/src/github.com/docker/docker
 [ -x hack/make.sh ] || usage