Update docker/README.md

Co-authored-by: MCJ Vasseur <[email protected]>

Author: as6325400

Diff for: docker/README.md

@@ -263,10 +263,10 @@ where `version` is the DOMjudge version to create the images for, e.g. `5.3.0`.
 
 To build domjudge with local sources, run
 ```bash
-  dpkp=<path to domjudge-packaging>
-  dj=<domjudge source directory>
-  tar --exclude-vcs -czf $dpkp/docker/domjudge.tar.gz -C $(dirname "$dj") $(basename "$dj")
-  cd $dpkp/docker
+  dj_packaging_dir=<path to domjudge-packaging>
+  dj_source_dir =<domjudge source directory>
+  tar --exclude-vcs -czf "${dj_packaging_dir}/docker/domjudge.tar.gz" -C $(dirname "$dj_source_dir") $(basename "$dj_source_dir")
+  cd ${dj_packaging_dir}/docker"
   docker build -t domjudge -f domserver/Dockerfile .
 ```
 Note that the source directory name has to match `domjudge-*`.