Skip chmod if DockerRunner has no user

What does this MR do?

For backward compatibility, do not set permissive permissions using chmod if DockerRunner isn't configured with the user.

See !63 (comment 2121286932)

Fixes https://gitlab.com/gitlab-org/security-products/analyzers/gemnasium/-/jobs/7874746393#L1364

Testing

• Draft: Test for integration-test@skip-chmod-if-... (semgrep!516 - closed)
• Draft: Test for integration-test@skip-chmod-if-... (gemnasium!873 - closed)

What are the relevant issue numbers?

None