unix/linux: upgrade Docker build container to Go 1.13

Change-Id: I1f47efb573ff3fa77afd9d1433726a908008c27d
Reviewed-on: https://go-review.googlesource.com/c/sys/+/193119
Run-TryBot: Matt Layher <mdlayher@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

unix/linux/Dockerfile

@@ -17,9 +17,9 @@ RUN git clone --branch v5.2 --depth 1 https://kernel.googlesource.com/pub/scm/li
 RUN git clone --branch release/2.30/master --depth 1 git://sourceware.org/git/glibc.git
 
 # Get Go
-ENV GOLANG_VERSION 1.13beta1
+ENV GOLANG_VERSION 1.13
 ENV GOLANG_DOWNLOAD_URL https://golang.org/dl/go$GOLANG_VERSION.linux-amd64.tar.gz
-ENV GOLANG_DOWNLOAD_SHA256 dbd131c92f381a5bc5ca1f0cfd942cb8be7d537007b6f412b5be41ff38a7d0d9
+ENV GOLANG_DOWNLOAD_SHA256 68a2297eb099d1a76097905a2ce334e3155004ec08cdea85f24527be3c48e856
 
 RUN curl -fsSL "$GOLANG_DOWNLOAD_URL" -o golang.tar.gz \
     && echo "$GOLANG_DOWNLOAD_SHA256  golang.tar.gz" | sha256sum -c - \