Fix docker build instructions missing build context

Signed-off-by: Rufus Wong <[email protected]>

Author: rcywongaa

docs/building.md

@@ -54,7 +54,7 @@ Build the Docker image with
 ```shell
 # Make sure to run this in the workspace directory
 # ROS_DISTRO can be humble|iron|rolling
-docker build -f src/ros2_rust/Dockerfile --build-arg "ROS_DISTRO=humble" -t ros2_rust_dev
+docker build -f src/ros2_rust/Dockerfile --build-arg "ROS_DISTRO=humble" -t ros2_rust_dev .
 ```
 
 and then run it with