document deploy rootless using docker compose

[devbgv]

Proposed changes
Adds a new how-to guide for deploying F5 NGINX Instance Manager (NIM) in a rootless Docker Compose environment, where all container processes run as the nms non-root user.

What changed and why:
The existing Docker deployment section covers the standard image-based Docker Compose flow. This PR adds a companion guide for teams with security hardening requirements—such as CIS benchmarks or internal least-privilege policies—that prevent running containers as root. The guide also documents the runtime configuration injection pattern used by this deployment, which allows NIM settings to be changed via environment variables and a stack restart, without rebuilding the Docker image. This is distinct from the standard deployment and warrants its own topic.

How the changes were made:

• The new file deploy-nim-rootless-docker-compose.md follows the established conventions of the existing Docker deployment topics: f5-content-type: how-to, f5-product: NIMNGR, toc: true, H2/H3 heading hierarchy, --- section separators, fenced code blocks with language identifiers, and {{< call-out >}} shortcodes for notes.
• weight: 150 places the page between the existing standard deployment guide (100) and the data plane guide (200) in the left-hand navigation.
• A {{< ref >}} cross-link to the standard Docker Compose guide is included in the "See also" section.
• _index.md description and f5-summary are updated to surface the new rootless option on the section landing page.
  Out of scope / follow-up considerations:
• This guide references startNIM.sh helper functions (set_nms_conf, set_nms_sm). If a reference topic for those functions is added in future, a cross-link should be added here.
• The f5-docs field is set to a placeholder (DOCS-NIM-ROOTLESS) and should be updated with the canonical JIRA ticket before merge.

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Checklist
Before sharing this pull request, I completed the following checklist:

• I read the Contributing guidelines (https://github.com/nginx/documentation/blob/main/CONTRIBUTING.md)
• My branch adheres to the Git conventions (https://github.com/nginx/documentation/blob/main/documentation/git-conventions.md)
• My content changes adhere to the F5 Technical Writing Style Guide (https://github.com/F5Docs/style-guide)
• If my changes involve potentially sensitive information^1, I have assessed the possible impact
• I have waited to ensure my changes pass tests, and addressed any discovered issues

[github-actions]

🎉 Thank you for your contribution! It appears you have not yet signed the F5 Contributor License Agreement (CLA), which is required for your changes to be incorporated into an F5 Open Source Software (OSS) project. Please kindly read the F5 CLA and reply on a new comment with the following text to agree:

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I have hereby read the F5 CLA and agree to its terms

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2 out of 3 committers have signed the CLA.
✅ (devbgv)[https://github.com/devbgv]
✅ (travisamartin)[https://github.com/travisamartin]
❌ @vamshi Krishna BGV
Vamshi Krishna BGV seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
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[devbgv]

I have hereby read the F5 CLA and agree to its terms

[github-actions]

✅ Deploy Preview will be available once build job completes!

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[JTorreG]

recheck

[travisamartin]

I opened a PR with my edits. Take a look: #1969

Issues addressed in separate PR