Add Docker container for local test of Python 2.6

It’s getting harder to find CI that can easily run 2.6 (without burning
CPU building it from source), so let’s just do it locally.  Hopefully
I’ll remember to run it before each release.

containers/python-2.6/Dockerfile

@@ -0,0 +1,9 @@
+FROM mrupgrade/deadsnakes:2.6
+RUN apt update
+RUN apt install -y -y build-essential python2.6-dev
+RUN pip install numpy==1.11.3
+RUN pip install argparse mock pytz unittest2
+RUN pip install https://github.com/brandon-rhodes/assay/archive/master.zip
+RUN pip install skyfield
+RUN echo 'assay --batch skyfield.tests' > /root/.bash_history
+CMD cd skyfield/ci && /bin/bash

containers/python-2.6/run

@@ -0,0 +1,18 @@
+#!/bin/bash
+
+set -ex
+
+if [ ! -f /usr/bin/docker ]
+then
+    echo Error: please apt install docker.io >&2
+    exit 2
+fi
+
+cd "$(dirname $0)"
+
+docker build -t skyfield-python-2.6 . 1>&2
+
+#     -v $PWD/../../../assay:/assay \
+exec docker run -ti \
+     -v $PWD/../..:/skyfield \
+     skyfield-python-2.6 "$@"