Add hostname flag to Docker docs

Fixes #4380

docker/README.md

@@ -16,9 +16,13 @@ Set environment variable `RESTIC_REPOSITORY` and map volume to directories and
 files like:
 
 ```
-docker run --rm -ti \
+docker run --rm --hostname my-host -ti \
     -v $HOME/.restic/passfile:/pass \
     -v $HOME/importantdirectory:/data \
     -e RESTIC_REPOSITORY=rest:https://user:pass@hostname/ \
     restic/restic -p /pass backup /data
 ```
+
+Restic relies on the hostname for various operations. Make sure to set a static
+hostname using `--hostname` when creating a Docker container, otherwise Docker
+will assign a random hostname each time.