*: Go 1.7 no longer supported and updated build image to 1.9

Dockerfile

-FROM golang:1.8.3-alpine
+FROM golang:1.9.2-alpine
 
 MAINTAINER Ed Rooth <ed.rooth@coreos.com>
 MAINTAINER Lucas Servén <lucas.serven@coreos.com>

scripts/check-go-version

@@ -9,17 +9,17 @@ MAJOR_GOVERSION=$( echo -n "$VERSION" | grep -o 'go1\.[0-9]' || true )
 FULL_GOVERSION=$( echo -n "$VERSION" | grep -o 'go1\.[0-9|\.]*' || true )
 
 # The list of unsupported major go versions.
-UNSUPPORTED=( "go1.0" "go1.1" "go1.2" "go1.3" "go1.4" "go1.5" "go1.6" )
+UNSUPPORTED=( "go1.0" "go1.1" "go1.2" "go1.3" "go1.4" "go1.5" "go1.6" "go1.7" )
 
 # Minor go versions which have known security vulnerabilities. Refuse to build with these.
 #
-# There aren't any security issues that impact dex in 1.7 but minor versions should be
-# added here later if they do have issues. 
+# There aren't any security issues that impact dex in current Go versions but minor versions
+# should be added here later if they do have issues.
 KNOWN_INSECURE=( )
 
 for V in "${UNSUPPORTED[@]}"; do
     if [ "$V" = "$MAJOR_GOVERSION" ]; then
-        >&2 echo "ERROR: dex requires Go version 1.7+. Please update your Go installation: https://golang.org/dl/" 
+        >&2 echo "ERROR: dex requires Go version 1.8+. Please update your Go installation: https://golang.org/dl/"
         exit 2
     fi
 done