unix: update Dockerfile to Linux 5.3 and use ubuntu 19.04 base image

rsync needs to be explicitly installed for 'make headers_install' to
work.

Change-Id: Icd3d6b3f004a7684b5ae32b4b7cab8d50631900d
Reviewed-on: https://go-review.googlesource.com/c/sys/+/195258
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matt Layher <mdlayher@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/linux/Dockerfile

@@ -1,18 +1,19 @@
-FROM ubuntu:18.10
+FROM ubuntu:19.04
 
 # Dependencies to get the git sources and go binaries
 RUN apt-get update && apt-get install -y  --no-install-recommends \
         ca-certificates \
         curl \
         git \
+        rsync \
     && apt-get clean \
     && rm -rf /var/lib/apt/lists/*
 
 # Get the git sources. If not cached, this takes O(5 minutes).
 WORKDIR /git
 RUN git config --global advice.detachedHead false
-# Linux Kernel: Released 07 July 2019
-RUN git clone --branch v5.2 --depth 1 https://kernel.googlesource.com/pub/scm/linux/kernel/git/torvalds/linux
+# Linux Kernel: Released 15 Sep 2019
+RUN git clone --branch v5.3 --depth 1 https://kernel.googlesource.com/pub/scm/linux/kernel/git/torvalds/linux
 # GNU C library: Released 02 Aug 2019 (we should try to get a secure way to clone this)
 RUN git clone --branch release/2.30/master --depth 1 git://sourceware.org/git/glibc.git