webots_ros2 | Webots ROS 2 packages | Robotics library

When you have multiple FROM commands, you're not "inheriting" both of their contents into the same image - you're doing a multi-stage build. This allows you to COPY from that stage specifying the --from option. By default, the last stage in your Dockerfile will be the target (so, in your example, you're only actually using the ros2 image. The webots image is not actually being used there.

You have two options here:

1. Copy just the files you need from the webots image using COPY --from=base

This will probably be hard and finicky. You'll need to copy all dependencies; and if they're acquired through your package manager (apt-get), you'll leave dpkg's local database inconsistent.

1. Copy one of the Dockerfiles and change their FROM

This will probably work fine as long as they both use the same base distribution. You can go into one of the project's repositories and grab their Dockerfile, rebuilding it from the other image - just change, for example, cyberbotics/webots:R2021a-ubuntu20.04's Dockerfile to have FROM niurover/ros2_foxy:latest. It may require tinkering with the other commands there, though.