Update readme

README.md

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 # ros2_debian
+
+The purpose of this repository is to provide docker images for building ros2_control on debian. 
+They are automatically built and pushed to https://github.com/ros-controls/ros2_rhel/pkgs/container/ros
+
+To manually build them, you can use the following commands:
+```bash
+docker build -t ros2_debian11_humble . -f Dockerfile.debian11 --build-arg ROS_DISTRO=humble
+docker build -t ros2_debian11_iron . -f Dockerfile.debian11 --build-arg ROS_DISTRO=iron
+docker build -t ros2_debian12_jazzy . -f Dockerfile.debian12 --build-arg ROS_DISTRO=jazzy
+docker build -t ros2_debian12_rolling . -f Dockerfile.debian12 --build-arg ROS_DISTRO=rolling
+```