Add nightly build systemd units.

docker/armv7l/README.md

@@ -1,6 +1,8 @@
 # [openstf/stf-armv7l](https://hub.docker.com/r/openstf/stf-armv7l/)
 
-An alternate docker image for `armv7l`.
+Functionally equivalent to the [openstf/stf](https://hub.docker.com/r/openstf/stf/) Docker image, but runs on `armv7l`.
+
+## Manual build
 
 To build the image, run the following commands from the repo root directory (**not** this directory) on an `armv7l` machine:
 
@@ -8,3 +10,86 @@ To build the image, run the following commands from the repo root directory (**n
 ARCH=armhf docker/armv7l/mkimage-alpine.sh
 docker build -f docker/armv7l/Dockerfile -t openstf/stf-armv7l:latest .
 ```
+
+Note that the build is very, very I/O and CPU-heavy. Don't run it daily on your microSD-backed board or the card will die.
+
+## Nightly build
+
+The following instructions have only been tested on a [Scaleway C1](https://www.scaleway.com/) running Arch Linux with the latest Docker-enabled kernel (currently 4.4.2).
+
+This folder includes [systemd](https://www.freedesktop.org/wiki/Software/systemd/) unit files for an automatic nightly build. The build takes such a long time that there's no point in doing it in real time via hooks.
+
+Once Scaleway tooling becomes a bit easier to approach we might automate the build server provisioning, but honestly it isn't that much work as you'll see.
+
+First, copy the unit files into your build machine's `/etc/systemd/system/` folder. Note that you may have to modify the docker image name inside the unit files if you're planning on pushing the nightly images.
+
+```bash
+cp stf-armv7l-* /etc/systemd/system/
+```
+
+Alternatively `scp` can be much easier depending on your setup:
+
+```bash
+scp stf-armv7l-* root@a.b.c.d:/etc/systemd/system
+```
+
+If you're upgrading the units or otherwise tweaking them, don't forget to let systemd know:
+
+```bash
+systemctl daemon-reload
+```
+
+Let's also make sure that docker and other deps are installed:
+
+```bash
+pacman -Sy
+pacman -S docker git
+```
+
+Some of the unit files require a dedicated build user to exist, so let's create that:
+
+```bash
+useradd --system --create-home -G docker stf-armv7l
+```
+
+Before enabling docker, you may wish to change the storage driver to `overlay`:
+
+```bash
+systemctl edit docker
+```
+
+Enter the following contents. Note that I also tend to always lock down docker's `--bip` so that it can't accidentally conflict with any other range (which I've personally experienced in a complex network). Feel free to remove it if you wish.
+
+```systemd
+[Service]
+ExecStart=
+ExecStart=/usr/bin/docker daemon -H fd:// -s overlay --bip 192.168.255.1/24
+```
+
+Now, enable and start `docker`:
+
+```bash
+systemctl enable docker
+systemctl start docker
+```
+
+If you want to push the built images, you should login to docker:
+
+```bash
+docker login
+```
+
+Now all you need to do is enable and start the timer.
+
+```bash
+systemctl enable stf-armv7l-publish-latest.timer
+systemctl start stf-armv7l-publish-latest.timer
+```
+
+You can also publish tags and other branches manually by:
+
+```bash
+systemctl start stf-armv7l-publish@v1.1.2
+```
+
+Note that the command won't return for a long long time. You can check progress by running `journalctl -f` if necessary.