devapp: update Dockerfile to add missing dependency
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 <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
Reviewed-by: Alexander Rakoczy <60c6d277a8bd81de7fdde19201bf9c58a3df08f4@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/build,golang/build,golang/build,golang/build