update go to 1.13 and alpine to 3.10

.travis.yml

@@ -6,7 +6,7 @@ dist: xenial
 
 matrix:
   include:
-    - go: '1.12.x'
+    - go: '1.13.x'
 
 env:
   global:

Dockerfile

@@ -1,11 +1,11 @@
-FROM golang:1.12.9-alpine
+FROM golang:1.13-alpine
 
 RUN apk add --no-cache --update alpine-sdk
 
 COPY . /go/src/github.com/dexidp/dex
 RUN cd /go/src/github.com/dexidp/dex && make release-binary
 
-FROM alpine:3.9
+FROM alpine:3.10
 # 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.

go.mod

@@ -60,3 +60,5 @@ require (
 	gopkg.in/ldap.v2 v2.5.1
 	gopkg.in/square/go-jose.v2 v2.3.1
 )
+
+go 1.13