Implement user script to step

What does this MR do?

Performs a migration of user scripts to CI Functions' steps.

Why was this MR needed?

Script-to-steps migration plan.

What's the best way to test this MR?

A new test job has been added: integration test (docker, steps). This is configured to allow_failure, as there's a few outstanding and known issues that prevent the full suite from passing.

What are the relevant issue numbers?

Closes step-runner#289 (closed)