Opt-in WITH_FFMPEG build arg installs Debian's ffmpeg in the backend image

Off by default, since it adds about 400 MB. Documented with the clip recipe
and in the configuration reference.

docs/code/nodes.md

@@ -150,9 +150,10 @@ def process(clip):
 ```
 
 Use it only for clips headed for such a panel. Browsers play the originals,
-and the result plays there too, just smaller. The Docker image does not ship
-`ffmpeg` yet, so the node runs only where `ffmpeg` is on the `PATH`, such as a
-[remote worker](workers.md).
+and the result plays there too, just smaller. The node needs `ffmpeg` on the
+`PATH`. The Docker image includes it only when built with
+[`WITH_FFMPEG=true`](../reference/configuration.md#image-options) in `.env`.
+Otherwise run the node on a [remote worker](workers.md) that has it.
 
 ## Printing