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 <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matt Layher <mdlayher@gmail.com>
diff --git a/unix/linux/Dockerfile b/unix/linux/Dockerfile
index 66fade2..e61ea62 100644
--- a/unix/linux/Dockerfile
+++ b/unix/linux/Dockerfile
@@ -21,9 +21,9 @@
 RUN git clone --branch release/2.31/master --depth 1 git://sourceware.org/git/glibc.git
 
 # Get Go
-ENV GOLANG_VERSION 1.14.4
+ENV GOLANG_VERSION 1.15rc1
 ENV GOLANG_DOWNLOAD_URL https://golang.org/dl/go$GOLANG_VERSION.linux-amd64.tar.gz
-ENV GOLANG_DOWNLOAD_SHA256 aed845e4185a0b2a3c3d5e1d0a35491702c55889192bb9c30e67a3de6849c067
+ENV GOLANG_DOWNLOAD_SHA256 ac092ebb92f88366786063e68a9531d5eccac51371f9becb128f064721731b2e
 
 RUN curl -fsSL "$GOLANG_DOWNLOAD_URL" -o golang.tar.gz \
     && echo "$GOLANG_DOWNLOAD_SHA256  golang.tar.gz" | sha256sum -c - \