unix: use glibc release branch in Dockerfile

It shouldn't make a difference for generating types and consts, but
let's use the glibc release branch anyway.

Fixes golang/go#29873

Change-Id: I15a3df250f5ae6181b2d036a10e0523ebe44ac3f
Reviewed-on: https://go-review.googlesource.com/c/158997
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/linux/Dockerfile

@@ -14,7 +14,7 @@ RUN git config --global advice.detachedHead false
 # Linux Kernel: Released 23 Dec 2018
 RUN git clone --branch v4.20 --depth 1 https://kernel.googlesource.com/pub/scm/linux/kernel/git/torvalds/linux
 # GNU C library: Released 01 Aug 2018 (we should try to get a secure way to clone this)
-RUN git clone --branch glibc-2.28 --depth 1 git://sourceware.org/git/glibc.git
+RUN git clone --branch release/2.28/master --depth 1 git://sourceware.org/git/glibc.git
 
 # Get Go
 ENV GOLANG_VERSION 1.12beta1