The number of CPUs exposed inside a Docker container is configurable:
As for starting observer, the error is because the container doesn’t have the necessary libraries (wxWidgets and dependencies). Even if you get those installed, seeing the window will require some additional plumbing.
An alternative that might be easier would be to connect a remote IEx session from your local machine to the BEAM inside the container; this previous discussion covers some of the details and has a good writeup at the very end of what you’ll need to do.
