Add containerd Docker storage backend

[elibosley]

Motivation

• Provide an alternative Docker image storage backend by adding support for Docker's containerd image store (containerd-snapshotter) as a selectable option in the UI and startup handling.

Description

• Add containerd as an option in the Docker settings UI by updating emhttp/plugins/dynamix.docker.manager/DockerSettings.page so users can choose it alongside overlay2 and native.
• Wire the new backend into daemon startup logic in etc/rc.d/rc.docker by selecting --feature containerd-snapshotter=true when DOCKER_BACKINGFS=containerd is configured and preserving existing overlay2/native behavior for other values.
• Document the new containerd backend in emhttp/languages/en_US/helptext.txt with a short description of what it enables for the Docker data-root.
• Changed code paths are limited to the three files above and follow the existing configuration and validation patterns.

Testing

• Ran bash -n etc/rc.d/rc.docker to validate shell syntax and it returned no syntax errors.
• Ran php -l emhttp/plugins/dynamix.docker.manager/DockerSettings.page to validate PHP syntax and it returned no syntax errors.
• Verified the UI select and help text strings are updated in the modified DockerSettings.page and helptext.txt files via a local diff check.

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Fixes #2583

Summary by CodeRabbit

• New Features
  • Added containerd as a selectable Docker storage driver option in Docker settings.
  • Enables Docker's containerd-snapshotter image store backend as an alternative to existing overlay2 and native drivers.
  • Updated help documentation to explain the new containerd storage option.

[coderabbitai]

Walkthrough

This pull request adds support for Docker's containerd image store backend (containerd-snapshotter) as a new storage driver option. Changes span help documentation, the Docker settings UI dropdown, and the runtime Docker configuration logic to handle the new storage driver selection.

Changes

Cohort / File(s)	Summary
containerd Storage Driver Support emhttp/languages/en_US/helptext.txt, emhttp/plugins/dynamix.docker.manager/DockerSettings.page, etc/rc.d/rc.docker	Added containerd as a selectable Docker storage driver option. Updated help text describing the containerd-snapshotter backend, added the option to the UI dropdown menu, and restructured the docker initialization script to handle containerd via a case statement that enables the containerd-snapshotter feature flag, while preserving the existing auto-detection fallback for other configurations.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Poem

🐰 A whisker-twitched commit hops into view,
Containerd snapshots, shiny and new!
From helptext to dropdowns, the Docker driver springs,
With three little files doing coordinated things! 🐇✨

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main change: adding containerd as a Docker storage backend option. It matches the core objective of the PR.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch codex/add-containerd-as-storage-backend

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[github-actions]

🔧 PR Test Plugin Available

A test plugin has been generated for this PR that includes the modified files.

Version: 2026.03.23.1245
Build: View Workflow Run

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📝 Modified Files:

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emhttp/languages/en_US/helptext.txt
emhttp/plugins/dynamix.docker.manager/DockerSettings.page
etc/rc.d/rc.docker

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[limetech]

LGTM

[github-actions]

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