Merge pull request #165 from cgwalters/doc-usage

docs/usage: New doc

docs/usage.md

@@ -0,0 +1,37 @@
+---
+nav_order: 3
+---
+
+# Operating system content and usage
+
+## Automatic updates enabled by default
+
+The base image here enables the
+[bootc-fetch-apply-updates.service](https://github.com/containers/bootc/blob/main/manpages-md-extra/bootc-fetch-apply-updates.service.md)
+systemd unit which automatically finds updated container images from the
+registry and will reboot into them.
+
+### Controlling automatic updates
+
+First, one can disable the timer entirely as part of a container build:
+
+```dockerfile
+RUN systemctl mask bootc-fetch-apply-updates.timer
+```
+
+Alternatively, one can use systemd "drop-ins" to override the timer
+(for example, to schedule updates for once a week), create a file
+like this, named e.g. `50-weekly.conf`:
+
+```systemd
+[Timer]
+# Clear previous timers
+OnBootSec= OnBootSec=1w OnUnitInactiveSec=1w
+```
+
+Then add it into your container:
+
+```dockerfile
+RUN mkdir -p /usr/lib/systemd/system/bootc-fetch-apply-updates.timer.d
+COPY 50-weekly.conf /usr/lib/systemd/system/bootc-fetch-apply-updates.timer.d
+```