${ROS_PKG} in not defined in DockerFiles

[schromya]

In the following docker files, the environment variable ${ROS_PKG} is referenced but not defined (lines 166-168):

• dockerfiles/ubuntu_22_humble_python_311_minimal.dockerfile
• dockerfiles/ubuntu_22_jazzy_python_311_minimal.dockerfile
• dockerfiles/ubuntu_24_jazzy_python_311_minimal.dockerfile

This not does raise any build errors, but should be fixed to increase clarity.