diff --git a/docker/README.md b/docker/README.md
index 038c78f7c..ecdf8a8b1 100644
--- a/docker/README.md
+++ b/docker/README.md
@@ -88,6 +88,7 @@ variables are available for configuration:
 * ``SYNAPSE_TURN_URIS``, set this variable to the coma-separated list of TURN
   uris to enable TURN for this homeserver.
 * ``SYNAPSE_TURN_SECRET``, set this to the TURN shared secret if required.
+* ``SYNAPSE_MAX_UPLOAD_SIZE``, the max upload size [default `10M`].
 
 Shared secrets, that will be initialized to random values if not set: