unix: update Dockerfile to Go 1.15

Change-Id: Ic3deb101db06686ccecdda679e715782df11b895
Reviewed-on: https://go-review.googlesource.com/c/sys/+/248137
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
diff --git a/unix/linux/Dockerfile b/unix/linux/Dockerfile
index e259371..e07db40 100644
--- a/unix/linux/Dockerfile
+++ b/unix/linux/Dockerfile
@@ -21,9 +21,9 @@
 RUN git clone --branch release/2.32/master --depth 1 git://sourceware.org/git/glibc.git
 
 # Get Go
-ENV GOLANG_VERSION 1.15rc2
+ENV GOLANG_VERSION 1.15
 ENV GOLANG_DOWNLOAD_URL https://golang.org/dl/go$GOLANG_VERSION.linux-amd64.tar.gz
-ENV GOLANG_DOWNLOAD_SHA256 f41a08f630f018bc5d9fd100bd9899516e4965356c78165157eb0eda9a17ac09
+ENV GOLANG_DOWNLOAD_SHA256 2d75848ac606061efe52a8068d0e647b35ce487a15bb52272c427df485193602
 
 RUN curl -fsSL "$GOLANG_DOWNLOAD_URL" -o golang.tar.gz \
     && echo "$GOLANG_DOWNLOAD_SHA256  golang.tar.gz" | sha256sum -c - \