unix: update Dockerfile to Go 1.15rc1
unix: update Dockerfile to Go 1.15rc1

This verifies that go tool cgo -godefs in Go 1.15 is able to generate all
consts and types in this package.

Change-Id: Ic63d15427f27527d7a6f8dc780ab560cde434bc8
Reviewed-on: https://go-review.googlesource.com/c/sys/+/244977
Run-TryBot: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Matt Layher <76528ff2e917253aa2211f68e4ca60b5b846dcf5@gmail.com>

golang/sys,multiplay/golang-sys,multiplay/golang-sys,multiplay/golang-sys,golang/sys,multiplay/golang-sys,golang/sys