python - What does "5 Illegal instruction" from bash script mean?

Question

The following script is an entrypoint to a docker image.

run.sh

#!/bin/bash

# wrapper function for docker entrypoint, allows:
# docker run -> runs the service
# docker run test -> runs the tests

set -e

export CLI='src/cli.py'

if [[ "$1" == "test"* ]]; then
    py.test tests
    exit $?
elif [[ "$1" == "junit"* ]]; then
    mkdir -p /junit
    py.test --junitxml=/junit/test-results.xml tests
    exit $?
fi;

python3 -u "$CLI" "$BACKEND"

What does this error mean?

./run.sh: line 20:     5 Illegal instruction     python3 -u "$CLI" "$BACKEND"

The container is up and running so I guess it fails sometimes, gets restarted and then works.