Allow passing args to docker user_script

[morgando]

user_script is the entrypoint that kicks off our testing logic inside Docker test clusters.

This change allows passing arguments to user_script. Existing usage remains unchanged, but optional arguments let us customize behavior when needed.

This prevents us from having to duplicate test scripts when we only need small variations in behavior.

Testing

Using

$ cat test_script.sh
#!/bin/bash

echo "Hi I was passed args '$@'"

1. Test with multiple arguments

$ comdb2/tests/docker/runit -s comdb2 -n 3 -a 0 -f test_script.sh --file-arg --my-opt --file-arg 1
...
c1        | READY
c1        | Hi I was passed args '0 --my-opt 1'

2. Test with no arguments

$ comdb2/tests/docker/runit -s comdb2 -n 3 -a 0 -f test_script.sh --file-arg --my-opt --file-arg 1
...
c1        | READY
c1        | Hi I was passed args ''

[roborivers]

Coding style check: Success ✓.
Smoke testing: Error ⚠.
Cbuild submission: Success ✓.
Regression testing: 1/626 tests failed ⚠.

The first 10 failing tests are:
sc_downgrade