util: Updated the Dockerfiles for Python3

For the next release of gem5, we are dropping support for Python2. The
Ubuntu 18.04 Docker images were running with Python2. This has been
updated.

It should be noted that there is, at present, no eligant solution to the
issue that older versions of Scons (such as that obtainable via APT in
Ubuntu 18.04) use Python2. Those wishing to compile with these Docker
Images should use
`/usr/bin/env python3 $(which scons) build/X86/gem5.op5`

Change-Id: Ic36ecc7196688daff21af2bb3a76381966f38f60
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/36595
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>

util/dockerfiles/ubuntu-18.04_all-dependencies/Dockerfile

@@ -30,5 +30,5 @@ 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 \
-    python-dev python python-six doxygen libboost-all-dev libhdf5-serial-dev \
-    python-pydot libpng-dev libelf-dev pkg-config
+    python3-dev python3 python3-six doxygen libboost-all-dev \
+    libhdf5-serial-dev python3-pydot libpng-dev libelf-dev pkg-config

util/dockerfiles/ubuntu-18.04_clang-version/Dockerfile

@@ -40,7 +40,7 @@ RUN apt -y update
 RUN apt -y upgrade
 RUN apt -y install git m4 scons zlib1g zlib1g-dev clang-${version} \
     libprotobuf-dev protobuf-compiler libprotoc-dev libgoogle-perftools-dev \
-    python-dev python python-six doxygen
+    python3-dev python3 python3-six doxygen
 
 RUN apt-get --purge -y remove gcc
 

util/dockerfiles/ubuntu-18.04_gcc-version/Dockerfile

@@ -37,7 +37,8 @@ RUN apt -y update
 RUN apt -y upgrade
 RUN apt -y install git m4 scons zlib1g zlib1g-dev gcc-multilib \
     libprotobuf-dev protobuf-compiler libprotoc-dev libgoogle-perftools-dev \
-    python-dev python python-six doxygen wget zip gcc-${version} g++-${version}
+    python3-dev python3 python3-six doxygen wget zip gcc-${version} \
+    g++-${version}
 
 RUN update-alternatives --install \
     /usr/bin/g++ g++ /usr/bin/g++-${version} 100

util/dockerfiles/ubuntu-18.04_min-dependencies/Dockerfile

@@ -28,5 +28,5 @@ FROM ubuntu:18.04
 
 RUN apt -y update
 RUN apt -y upgrade
-RUN apt -y install build-essential scons zlib1g-dev m4 python-dev python \
-    python-six
+RUN apt -y install build-essential scons zlib1g-dev m4 python3-dev python3 \
+    python3-six