devapp: update Dockerfile to add missing dependency

Also updates the base image to Go 1.13 and removes the explicit
setting of GOPROXY as it’s not needed anymore in 1.13.

Change-Id: Ifa31fa2e7b1fb0f62f945c4dcea82900437dee85
Reviewed-on: https://go-review.googlesource.com/c/build/+/193878
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Alexander Rakoczy <alex@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/devapp/Dockerfile b/devapp/Dockerfile
index c785a4c..0180ee0 100644
--- a/devapp/Dockerfile
+++ b/devapp/Dockerfile
@@ -2,11 +2,10 @@
 # Use of this source code is governed by a BSD-style
 # license that can be found in the LICENSE file.
 
-FROM golang:1.12 AS builder
+FROM golang:1.13 AS builder
 LABEL maintainer "golang-dev@googlegroups.com"
 
 ENV GO111MODULE=on
-ENV GOPROXY=https://proxy.golang.org
 
 RUN mkdir /gocache
 ENV GOCACHE /gocache
@@ -25,6 +24,7 @@
 RUN go install golang.org/x/build/cmd/pubsubhelper/pubsubtypes
 COPY autocertcache /go/src/golang.org/x/build/autocertcache
 RUN go install golang.org/x/build/autocertcache
+COPY internal /go/src/golang.org/x/build/internal
 COPY maintner /go/src/golang.org/x/build/maintner
 RUN go install golang.org/x/build/maintner/godata