Fix /usr/lib/libnss_wrapper.so not found error in Docker image

[simu]

#402 updates the Docker base image to Debian bullseye, which changes the location of libnss_wrapper.so from /usr/lib/libnss_wrapper.soto /usr/lib/x86_64-linux-gnu/libnss_wrapper.so. Our tests didn't catch this, because we didn't run the Docker image after building it.

This PR adds a few things:

1. A step in the docker image build Github Action which runs the freshly built image
2. A check that the configured libnss_wrapper.so path exists in /usr/local/bin/entrypoint.sh of the Docker image. This should ensure that the GitHub Action fails when the location of the library changes
3. Update the entrypoint script to use the correct path for Debian bullseye

Checklist

• Keep pull requests small so they can be easily reviewed.
• Update tests.
• Categorize the PR by setting a good title and adding one of the labels:
  bug, enhancement, documentation, change, breaking, dependency
  as they show up in the changelog