Remove unsupported container option from copilot-setup-steps.yml documentation (#57844)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: timrogers <116134+timrogers@users.noreply.github.com>
Co-authored-by: hubwriter <hubwriter@github.com>

Author: 3 people

content/copilot/how-tos/use-copilot-agents/coding-agent/customize-the-agent-environment.md

@@ -88,7 +88,6 @@ In your `copilot-setup-steps.yml` file, you can only customize the following set
 * `steps` (see above)
 * `permissions` (see above)
 * `runs-on` (see below)
-* `container `
 * `services`
 * `snapshot`
 * `timeout-minutes` (maximum value: `59`)