doc/source/docker.rst: Fix formatting

doc/source/docker.rst

@@ -13,6 +13,8 @@ We recommend using the
 which automates the necessary container setup. You can download it and make
 it executable like this:
 
+.. code:: bash
+
   mkdir -p ~/.local/bin
   curl --get https://gitlab.com/BuildStream/buildstream/raw/master/contrib/bst-here > ~/.local/bin/bst-here
   chmod +x ~/.local/bin/bst-here