image: resolve --image via the Docker daemon API instead of the CLI

[congwang-mk]

Summary

• --image takes a local Docker image reference again (python:3.12-slim, a digest, or an image id), not an archive path.
• Sandlock talks to the Docker daemon directly through its HTTP API (the bollard crate) instead of shelling out to the docker CLI. The old path ran docker create + docker export + docker inspect + docker rm as subprocesses; those are gone, along with the tar subprocess that piped the export. The docker binary no longer needs to be on PATH.
• A running daemon with an accessible socket is still required. --image pings the daemon up front and fails early with a clear message when it is unreachable, rather than partway through sandbox setup.
• Image lookup is local only: inspect_image never pulls from a registry, and a missing image surfaces the daemon's 404 as image not found in local Docker storage.
• Rootfs is materialized the same way docker export works: a throwaway stopped container is created, its flattened filesystem is streamed out with export_container into a temp tar (bounded memory, never held whole), then unpacked. The container is always removed afterward, even when the export fails.
• Because docker export yields an already merged filesystem, the per-layer, gzip, and AUFS/OCI whiteout handling is gone. Only the hard-link fixups remain (the tar crate resolves link targets against the process cwd rather than the destination root).
• Cache key is the image content id, stored at ~/.cache/sandlock/images/<image-id>/rootfs/ with a .complete marker distinguishing a finished extraction from an interrupted one. Warm-cache invocations skip the daemon export entirely.
• Deps: drop flate2 and sha2; add bollard and futures-util; enable tokio's fs feature.

Also works with Podman: point DOCKER_HOST at podman system service's socket, since bollard speaks the Docker API.

Builds on #73.

Test plan

• cargo test -p sandlock-core image:: (5 image unit tests: hard-link forward references, regular files, default cmd, id sanitization) all green
• cargo test -p sandlock-cli (26 tests) green
• Manual: sandlock run --image python:3.11-slim -- python3 -c 'import sys; print(sys.version.split()[0])' returns 3.11.15
• Manual: sandlock run --image ubuntu:22.04 -- cat /etc/os-release returns Ubuntu 22.04.5 LTS
• Warm cache hit on second invocation (~0.19s, skips export)
• Missing image gives a clear local-storage 404 error
• Unreachable daemon (DOCKER_HOST pointed at a bogus socket) fails early before any sandbox setup