Update alpine to 3.8

Dockerfile

@@ -5,7 +5,7 @@ RUN apk add --no-cache --update alpine-sdk
 COPY . /go/src/github.com/coreos/dex
 RUN cd /go/src/github.com/coreos/dex && make release-binary
 
-FROM alpine:3.7
+FROM alpine:3.8
 # Dex connectors, such as GitHub and Google logins require root certificates.
 # Proper installations should manage those certificates, but it's a bad user
 # experience when this doesn't work out of the box.