Rename the build directory in the e2e docker doc

To avoid conflicting with the “host” build/ directory. This way we don’t
need to remove/empty the build/ directory beforehand, etc.

doc/developer.rst

@@ -71,7 +71,7 @@ created):
 .. code-block:: bash
   :caption: Configure and build biboumi from inside the container
 
-  docker exec biboumi-e2e sh -c "cd biboumi && mkdir build/ && cd build/ && cmake .."
+  docker exec biboumi-e2e sh -c "cd biboumi && mkdir docker-build/ && cd docker-build/ && cmake .."
 
 This is needed (only once), because if you configure it from your host
 machine, then the paths generated by cmake will be all wrong when you try
@@ -80,7 +80,7 @@ to compile from inside the container and nothing will work.
 .. code-block:: bash
   :caption: Re-compile and run the test suite inside the container
 
-  docker exec biboumi-e2e sh -c "cd biboumi/build && make e2e"
+  docker exec biboumi-e2e sh -c "cd biboumi/docker-build && make e2e"
 
 This should now build everything correctly, and run the test suite. If you
 want to re-run it again after you edited something in your source tree,