doc: Add instructions for running Tutor on Podman

https://podman.io/ is meant to be a drop-in replacement for Docker.
Thus, with some tweaking to the installation environment, it appears
to be perfectly feasible to run Tutor in a Docker-less environment
that only has Podman and podman-compose installed.

Add installation instructions for doing just that.

docs/install.rst

@@ -6,7 +6,7 @@ Installation
 Requirements
 ------------
 
-The only prerequisite for running this is a working docker installation. Both docker and docker-compose are required. Follow the instructions from the official documentation:
+The only prerequisite for running this is a working Docker installation. You'll need both the ``docker`` and ``docker-compose`` commands in your system ``$PATH``. Follow the instructions from the official documentation:
 
 - `Docker <https://docs.docker.com/engine/installation/>`_: minimum supported version is 18.06.0.
 - `Docker compose <https://docs.docker.com/compose/install/>`_
@@ -76,4 +76,6 @@ If you are running zsh, run instead::
 
 After opening a new shell, you can test auto-completion by typing::
 
-    tutor <tab><tab>
+    tutor <tab><tab>
+
+.. include:: podman.rst

docs/podman.rst

@@ -0,0 +1,51 @@
+.. _podman:
+
+Running Tutor with Podman
+-------------------------
+
+You have the option of running Tutor with `Podman <https://podman.io/>`__, instead of the native Docker tools. This has some practical advantages: it does not require a running Docker daemon, and it enables you to run and build Docker images without depending on any system component running ``root``. As such, it is particularly useful for building Tutor images from CI pipelines.
+
+The ``podman`` CLI aims to be fully compatible with the ``docker`` CLI, and ``podman-compose`` is a fully-compatible alias of ``docker-compose``. This means that you can use both together with Tutor, without making any changes to Tutor itself.
+
+.. warning::
+   You should not attempt to run Tutor with Podman on a system that already has native ``docker`` and ``docker-compose`` installed. If you want to switch to ``podman`` and ``podman-compose`` using the aliases described here, uninstall the native Docker packages first. 
+
+
+Enabling Podman
+^^^^^^^^^^^^^^^
+
+Podman is supported on a variety of development platforms, see the `installation instructions <https://podman.io/getting-started/installation>`_ for details.
+
+Once you have installed Podman and its dependencies on the platform of your choice, you'll need to make sure that its ``podman`` binary, usually installed as ``/usr/bin/podman``, is aliased to ``docker``, and is included as such in your system ``$PATH``. On some CentOS and Fedora releases you can install a package named ``podman-docker`` to do this for you, but on other platforms you'll need to take of this yourself.
+
+- If ``$HOME/bin`` is in your ``$PATH``, you can create a symbolic link there::
+
+    ln -s $(which podman) $HOME/bin/docker
+
+- If you want to instead make ``docker`` a system-wide alias for ``podman``, you can create your symlink in ``/usr/local/bin``, an action that normally requires ``root`` privileges::
+
+    sudo ln -s $(which podman) /usr/local/bin/docker
+
+
+Enabling podman-compose
+^^^^^^^^^^^^^^^^^^^^^^^
+
+``podman-compose`` is available as a package from PyPI, and can thus be installed with ``pip``. See `its README <https://github.com/containers/podman-compose/blob/devel/README.md>`_ for installation instructions. Note that if you have installed Tutor in its own virtualenv, you'll need to run ``pip install podman-compose`` in that same virtualenv.
+
+Once installed, you'll again need to create a symbolic link that aliases ``docker-compose`` to ``podman-compose``.
+
+- If you run Tutor and ``podman-compose`` in a virtualenv, create the symlink in that virtualenv's ``bin`` directory: activate the virtualenv, then run::
+
+    ln -s $(which podman-compose) $(dirname $(which podman-compose))/docker-compose
+
+- If you do not, create the symlink in ``/usr/local/bin``, using ``root`` privileges::
+
+    sudo ln -s $(which podman-compose) /usr/local/bin/docker-compose
+
+
+Verifying your environment
+^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Once you have configured your symbolic links as described, you should be able to run ``docker version`` and ``docker-compose --help`` and their output should agree, respectively, with ``podman version`` and ``podman-compose --help``.
+
+After that, you should be able to use ``tutor local``, ``tutor build``, and other commands as if you had installed the native Docker tools.