*: only use docker when releasing, update to Go 1.8, remove aci scripts

This change modifies our release process to only require Docker
when building a release and updates our released binary to use Go
1.8. It also removes our .aci scripts, which we've not been
regularly building.

A nice consequence is that OSX users can now build a release image.

scripts/docker-build

@@ -0,0 +1,15 @@
+#!/bin/bash -e
+
+mkdir -p _output/bin
+
+sudo docker run \
+    --cidfile=cid \
+    -v $PWD:/go/src/github.com/coreos/dex:ro \
+    -w /go/src/github.com/coreos/dex \
+    golang:1.8.0-alpine \
+    /bin/sh -x -c \
+    'apk add --no-cache --update alpine-sdk && make release-binary'
+
+sudo docker cp $( cat cid ):/go/bin/dex _output/bin/dex
+sudo docker rm $( cat cid )
+sudo rm cid