Start adding support for multiple Python versions

Twiddle Python setup to use a virtualenv, and to accept an argument to
select which Python version to use. Split Python setup to a separate
provisioning step so that the other bits can live in a shared image.

This seems to be close to "good enough" to make Drake actually build,
mostly by virtue of shoving a bunch of symlinks to the appropriate
Python version stuff straight into /usr.

Dockerfile

@@ -1,13 +1,22 @@
+ARG PLATFORM=ubuntu:18.04
+
 # -----------------------------------------------------------------------------
 # Create a base provisioned image
 # -----------------------------------------------------------------------------
 
-FROM ubuntu:18.04 AS base
+FROM ${PLATFORM} AS base
 
-ADD image/provision.sh /image/
+ARG PYTHON=3
 
 ENV DEBIAN_FRONTEND=noninteractive
-RUN /image/provision.sh
+
+ADD image/provision-base.sh /image/
+
+RUN /image/provision-base.sh
+
+ADD image/provision-python.sh /image/
+
+RUN /image/provision-python.sh ${PYTHON}
 
 # -----------------------------------------------------------------------------
 # Build Drake's dependencies

image/provision.sh → image/provision-base.sh

@@ -3,14 +3,11 @@
 BAZEL_VERSION=4.1.0
 BAZEL_ROOT=https://github.com/bazelbuild/bazel/releases/download
 
-ln -s /usr/bin/python3 /usr/bin/python
-
 # Install prerequisites
 apt-get -y update
 
 apt-get -y install --no-install-recommends \
     autoconf automake default-jdk \
-    python3-dev libpython3-dev python3-pip \
     gcc g++ gfortran libgfortran-7-dev \
     libclang-9-dev clang-format-9 \
     git cmake ninja-build pkg-config \
@@ -21,13 +18,6 @@ apt-get -y install --no-install-recommends \
     libgl1-mesa-dev libxt-dev \
     opencl-headers ocl-icd-opencl-dev
 
-pip3 install \
-    pyyaml \
-    semantic-version \
-    setuptools \
-    wheel \
-    auditwheel
-
 # Install bazel
 cd /tmp
 wget ${BAZEL_ROOT}/${BAZEL_VERSION}/bazel-${BAZEL_VERSION}-installer-linux-x86_64.sh

image/provision-python.sh

@@ -0,0 +1,21 @@
+#!/bin/bash -e
+
+PYTHON=python${1:-3}
+
+# Set up Python environment and install Python prerequisites
+apt-get -y install --no-install-recommends \
+    ${PYTHON}-dev lib${PYTHON}-dev ${PYTHON}-venv
+
+${PYTHON} -m venv /usr/local
+
+pip install \
+    pyyaml \
+    semantic-version \
+    setuptools \
+    wheel \
+    auditwheel
+
+ln -s /usr/bin/${PYTHON}-config /usr/bin/python3-config
+ln -s /usr/bin/${PYTHON}-config /usr/local/bin/python-config
+ln -s /usr/local/bin/python /usr/bin/python
+ln -s /usr/include/${PYTHON}m /usr/local/include/