misc: Added Dockerfile for Ubuntu 20.04

This Dockerfile creates an image which simulates an Ubuntu 20.04
environment. Unlike the Ubuntu 18.04 Dockerfile, this does not use
Python2. It uses exclusively Python3. Ubuntu 20.04 has Python3 installed
by default. The image this Dockerfile creates can be obtained from
"gcr.io/gem5-test/ubuntu-20.04_all-dependencies". To pull:

docker pull gcr.io/gem5-test/ubuntu-20.04_all-dependencies

Change-Id: I73b51028e0d6a3198aa6e7b1906d20ed6eb6c815
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/28889
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
diff --git a/util/dockerfiles/ubuntu-20.04_all-dependencies/Dockerfile b/util/dockerfiles/ubuntu-20.04_all-dependencies/Dockerfile
new file mode 100644
index 0000000..814ef75
--- /dev/null
+++ b/util/dockerfiles/ubuntu-20.04_all-dependencies/Dockerfile
@@ -0,0 +1,35 @@
+# Copyright (c) 2020 The Regents of the University of California
+# All Rights Reserved.
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions are
+# met: redistributions of source code must retain the above copyright
+# notice, this list of conditions and the following disclaimer;
+# redistributions in binary form must reproduce the above copyright
+# notice, this list of conditions and the following disclaimer in the
+# documentation and/or other materials provided with the distribution;
+# neither the name of the copyright holders nor the names of its
+# contributors may be used to endorse or promote products derived from
+# this software without specific prior written permission.
+#
+# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+# "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+# LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+# A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+# OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+# SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+# LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+# DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+# THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+# (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+# OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+FROM ubuntu:20.04
+
+ENV DEBIAN_FRONTEND=noninteractive
+RUN apt -y update
+RUN apt -y upgrade
+RUN apt -y install build-essential git m4 scons zlib1g zlib1g-dev \
+    libprotobuf-dev protobuf-compiler libprotoc-dev libgoogle-perftools-dev \
+    python3-dev python3-six python-is-python3 doxygen libboost-all-dev \
+    libhdf5-serial-dev python3-pydot libpng-dev