docs: Document the test-install tooling.

docs/development/index.rst

@@ -11,3 +11,4 @@ Development environment
    Using the development environment <using>
    Developing remotely <remote>
    Authentication in the development environment <authentication>
+   Testing the installer <test-install>

docs/development/test-install.md

@@ -0,0 +1,127 @@
+# Testing the installer
+
+Zulip's install process is tested as part of [its continuous
+integrations suite][CI], but that only tests the most common
+configurations; when making changes to more complicated [installation
+options][installer-docs], Zulip provides tooling to repeatedly test
+the installation process in a clean environment each time.
+
+[CI]: https://github.com/zulip/zulip/actions/workflows/production-suite.yml?query=branch%3Amaster
+[installer-docs]: ../production/install.md
+
+## Configuring
+
+Using the test installer framework requires a Linux operating system;
+it will not work on WSL, for instance.  It requires at least 3G of
+RAM, in order to accommodate the VMs and the steps which build the
+release assets.
+
+To begin, install the LXC toolchain:
+```
+sudo apt-get install lxc lxc-utils
+```
+
+All LXC commands (and hence many parts of the test installer) will
+need to be run as root.
+
+## Running a test install
+
+### Build and unpack a release tarball
+
+You only need to do this step once per time you work on a set of
+changes, to refresh the package that the installer uses. The installer
+doesn't work cleanly out of a source checkout; it wants a release
+checkout, so we build a tarball of one of those first:
+```
+./tools/build-release-tarball test-installer
+```
+
+This will produce a file in /tmp, which it will print out the path to
+as the last step; for example,
+`/tmp/tmp.fepqqNBWxp/zulip-server-test-installer.tar.gz`
+
+Next, unpack that file into a local directory; we will make any
+changes we want in our source checkout and copy them into this
+directory. The test installer needs the release directory to be named
+`zulip-server`, so we rename it and move it appropriately.  In the
+first line, you'll need to substitute the actual path that you got for
+the tarball, above:
+```
+tar xzf /tmp/tmp.fepqqNBWxp/zulip-server-test-installer.tar.gz
+mkdir zulip-test-installer
+mv zulip-server-test-installer zulip-test-installer/zulip-server
+```
+
+You should delete and re-create this `zulip-test-installer` directory
+(using these steps) if you are working on a different installer
+branch, or a significant time has passed since you last used it.
+
+### Test an install
+
+The `test-install` tooling takes a distribution release name
+(e.g. "focal" or "bionic"), the path to an unpacked release directory
+or tarball, and then any of the normal options you want to pass down
+into the installer.
+
+For example, to test an install onto Ubuntu 20.04 "Focal", we might
+call:
+```
+sudo ./tools/test-install/install \
+  -r focal \
+  ./zulip-test-installer/ \
+  --hostname=zulip.example.net \
+  --email=username@example.net
+```
+
+The first time you run this command for a given distribution, it will
+build a "base" image for that to use on subsequent times; this will
+take a while.
+
+### See running containers after installation
+
+Regardless of if the install succeeds or fails, it will stay running
+so you can inspect it. You can see all of the containers which are
+running, and their randomly-generated names, by running:
+```
+sudo lxc-ls -f
+```
+
+### Connect to a running container
+
+After using `lxc-ls` to list containers, you can choose one of them
+and connect to its terminal:
+```
+sudo lxc-attach --clear-env -n zulip-install-focal-PUvff
+```
+
+### Stopping and destroying containers
+
+To destroy all containers (but leave the base containers, which speed
+up the initial install):
+```
+sudo ./tools/test-install/destroy-all -f
+```
+
+To destroy just one container:
+```
+sudo lxc-destroy -f -n zulip-install-focal-PUvff
+```
+
+
+
+### Iterating on the installer
+
+Iterate on the installer by making changes to your source tree,
+copying them into the release directory, and re-running the installer,
+which will start up a new container. Here, we update just the
+`scripts` and `puppet` directories of the release directory:
+```
+rsync -az scripts puppet zulip-test-installer/zulip-server/
+
+sudo ./tools/test-install/install \
+ -r focal \
+ ./zulip-test-installer/ \
+ --hostname=zulip.example.net \
+ --email=username@example.net
+```
+