What packages are in different ros dockerhub images?

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I believe the naming of these images -- and their contents -- follows the variants defined in REP-150: ROS Melodic and Newer Metapackages (for ROS 2, it's REP-2001: ROS 2 Variants).

For ros_core for instance:

The ros_core metapackage composes the core communication protocols. It may not contain any GUI dependencies.

- ros_core:
     packages: [catkin, class_loader, cmake_modules, common_msgs, gencpp,
                geneus, genlisp, genmsg, gennodejs, genpy, message_generation,
                message_runtime, pluginlib, ros, ros_comm,
                rosbag_migration_rule, rosconsole, rosconsole_bridge,
                roscpp_core, rosgraph_msgs, roslisp, rospack, std_msgs,
                std_srvs]

that's why you only see apt-get install [..] ros-noetic-perception in the Dockerfile.

It just installs the perception variant.

The focal and buster suffixes are indeed to differentiate between the different OS supported by a particular ROS version.

ROS Noetic supports both Ubuntu Focal and Debian Buster. But the ros_core variant is part of both of those.

So to differentiate between them, the OS code name was added, which makes it

The ros_core variant of ROS Noetic on Ubuntu Focal

and similar for Debian Buster.

Those Dockerfiles are not written by hand, but generated using the scripts in the osrf/docker_images repository. Those scripts parse a large .yaml file which contains information on all the different OS + ROS version + variant combinations. See osrf/docker_images/ros/manifest.yaml for the information used to generate the ROS Noetic images.

The tags for each image are also generated based on this information:

noetic:
  eol: 2025-05
  os_names:
    ubuntu:
      os_code_names:
        focal:
          <<: *DEFAULT_ROS1
          archs:
            - amd64
            - arm32v7
            - arm64v8
          tag_names:
            ros-core:
              aliases:
                - "$release_name-ros-core"
                - "$release_name-ros-core-$os_code_name"
    debian:
      [..]

See the aliases key, which is responsible for the noetic-ros-core, noetic-ros-core-focal images you found (and which indeed contain the same packages).