docs(readme): document Docker as the only backend

[bilby91]

Follow-up to #121 (which removed the Apple Containers CI jobs). Strips the applecontainer and podman backend documentation from the README, leaving Docker as the only documented backend.

What changed

• Backends — dropped the runtime/applecontainer and runtime/podman bullets and the "All three backends" line. Kept the pluggability statement, since the engine, feature pipeline, lifecycle and compose paths are written against runtime.Runtime rather than against Docker directly.
• Apple-container gotchas — the whole ~40-line section removed.
• Spec compliance — the compose row no longer claims the native orchestrator "works against both runtime/docker and runtime/applecontainer".
• Install → Requires — the three-way "a container backend, one of:" list collapsed to a single Docker bullet.
• Quick start — removed the "swap the backend import" applecontainer snippet.
• Sub-packages — removed the runtime/applecontainer bullet.
• Tests — removed the apple integration-test paragraph.
• Out of scope — "Kubernetes / podman drivers" → "Kubernetes drivers".

Docs only, no code changes. grep -i "apple\|podman\|criu\|checkpoint\|swift\|darwin\|macos" README.md is now empty. 318 → 236 lines.

Deliberately out of scope

design/ is untouched — runtime-applecontainer.md, podman-backend.md and checkpoint-restore.md are design records of decisions that were actually made, and design/README.md still indexes them. No code was removed: runtime/applecontainer/, applecontainer-bridge/ and runtime/podman/ all still ship.

Two things reviewers should know

Podman is not in the same state as applecontainer. Its CI job is still alive — test-integration-podman at .github/workflows/ci.yml:103 runs real Podman + CRIU checkpoint/restore in a privileged quay.io/podman/stable container, and it was untouched by #121. So after this PR the README documents less than CI actually covers. That is intentional per the request, not an oversight.

Checkpoint/restore lost its only README mention. Engine.Checkpoint, Engine.Restore, Engine.CheckpointProject and Engine.RestoreProject (checkpoint.go:66,109, checkpoint_project.go:110,193) are public root-package API, and runtime/podman is the only backend implementing runtime.CheckpointRuntime. The podman bullet was the README's only explanation of what those four methods require; that is now documented only under design/checkpoint-restore.md. A backend-neutral line under API surface ("checkpoint/restore requires a backend implementing runtime.CheckpointRuntime") would close that gap without naming Podman — happy to add it if wanted.

🤖 Generated with Claude Code

Summary by CodeRabbit

• Documentation
  • Updated setup and usage guidance to focus on the Docker backend.
  • Removed Apple-container and Podman backend references and examples.
  • Clarified Docker-specific requirements, API packages, quick-start steps, and integration testing.
  • Updated scope documentation to list Kubernetes drivers as out of scope.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 51a3d3c2-e45d-4027-bf33-fc608303193f

📥 Commits

Reviewing files that changed from the base of the PR and between 71e2e09 and 547c072.

📒 Files selected for processing (1)

• README.md

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📝 Walkthrough

Walkthrough

README.md now documents Docker as the supported container backend. It removes Apple-container and Podman references from backend, compose, requirements, quick-start, API, and integration-test sections.

Changes

Docker-only README documentation

Layer / File(s)	Summary
Backend scope and requirements README.md	Backend, compose, compliance, and installation sections now describe Docker and supported runtime abstractions.
Usage and integration documentation README.md	Quick-start, API, and integration-test documentation no longer describes Apple-container support.

Estimated code review effort: 1 (Trivial) | ~3 minutes

Merge Risk: ⚪ Minimal · up to 547c0

This is a localized documentation-only change and no actionable merge-blocking risk remains after normal checks and review.

Poem

I am a rabbit with Docker in sight
The README now keeps its scope tight
Apple examples hop away
Compose notes follow the same way
Clear paths lead through docs tonight

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: updating the README to document Docker as the only backend.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

Full details: Docstring Coverage

Explanation

No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0 files. (1 skipped: 1 unsupported.)

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch docs/readme-docker-only

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_{Comment @coderabbitai help to get the list of available commands.}

[dap-code-review-by-crunchloop]

Code Review — head 547c072

Reviewed the checked-out PR head 547c07277022e056ad6b5d537d3d0a5e2185b259 against base 71e2e099f5597c09cf3508d78726ff6b60e17471; the range is one commit changing one file (README.md, 15 additions/97 deletions). Read the base-ref engineering directives and CONTRIBUTING.md, the complete changed README, the Makefile and CI/test sources that consume its integration-test instructions, and the checkpoint/backend sources affected by the removed backend documentation. Findings were recorded for the inaccurate multi-backend integration-test instructions and the loss of checkpoint API/backend requirements. No code or tests were executed. Coverage gap: the directives reference PRD.md, but that file does not exist at the governing base commit, so no PRD claims could be checked. No design file was changed; relevant backend/checkpoint design records were inspected for contract context. Replies typed into inline comment threads will not reach this review; responses should be made in the pull request conversation.

Medium / Low

[MEDIUM] [D12] The integration-test instructions now misidentify a multi-backend target as Docker-only

• Anchor: README.md:210-219

• Witness: Makefile:13-15 implements make test-integration as go test -tags=integration ... ./test/integration/..., and the real source includes Linux Podman tests gated by PODMAN_SOCKET (test/integration/podman_checkpoint_restore_test.go:1-18) plus darwin/arm64 Apple-container tests (test/integration/applecontainer_image_source_test.go:1-31). With PODMAN_SOCKET set and no Docker daemon, or on macOS with the Apple daemon running, this target still runs real non-Docker tests; nevertheless the README says the target is “integration tests against real Docker” and that the suite is skipped when a Docker daemon is unavailable (README.md:210-219).

• Consumer: A contributor following README.md:210-219 and invoking make test-integration receives the target behavior from Makefile:13-15; the command can exercise Podman or Apple-container and can fail or consume resources even when Docker is unavailable, contrary to the stated prerequisite and skip behavior.

• Fix: Make the README describe the platform/backend gates (Docker tests, Linux Podman tests when PODMAN_SOCKET is set, and darwin/arm64 Apple-container tests when their daemon is available), or change the Make target to explicitly select only the documented Docker suite and retain separate commands for the other backends.

Commented inline

• [MEDIUM] [D11] Removing the backend section makes the public checkpoint APIs undiscoverable and directs users to an incapable backend — README.md:40

Verdict

ADVISORY — findings worth reading, none of them blocking.

[dap-code-review-by-crunchloop]

[MEDIUM] [D11] Removing the backend section makes the public checkpoint APIs undiscoverable and directs users to an incapable backend

• Witness: The new README contains no checkpoint, restore, or podman reference (the repository grep over README.md returns no matches), while it presents runtime/docker as the documented backend at README.md:31-36. The public source exposes Engine.Checkpoint and Engine.Restore at checkpoint.go:55-66 and checkpoint.go:93-109, but those methods gate on runtime.CheckpointRuntime; the runtime contract explicitly says only runtime/podman implements it and Docker does not (runtime/runtime.go:88-99), and Docker's Capabilities leaves Checkpoint false (runtime/docker/compose_primitives.go:196-204).
• Consumer: A user following the README's only backend setup and then looking for the public checkpoint feature is sent to Docker, where Engine.Checkpoint/Restore return ErrCheckpointUnsupported; the README no longer provides even the capability requirement or a path to the implementing backend, despite these APIs being part of the shipped root package.
• Fix: Keep the Docker-only backend overview if required, but add a backend-neutral note in the API section that Engine.Checkpoint, Restore, CheckpointProject, and RestoreProject require a runtime implementing runtime.CheckpointRuntime (and link the capability/backend documentation), or retain the minimal Podman prerequisite needed to make those public APIs usable.