[feat request/enhancement] support json format output: udocker ps --format json

[AClon314]

This make other scripts can use the CLI output directly without writing additional format parser for udocker.
like podman images --format json can give json output:

[
    {
        "Id": "0fd2ff36baf788cdf00a402ac50865af6dfcb73a9e7ff264e6a2b7334b9e5711",
        "ParentId": "",
        "RepoTags": null,
        "RepoDigests": [
            "ghcr.io/prefix-dev/pixi@sha256:3b277f42e137adb3421b61a4b169eef2141b126f57db61eb368098ef0aab39ed",
            "ghcr.io/prefix-dev/pixi@sha256:d2b1b423bd60adb9ce8b739ba065409ac0024611fe8439944111a5c7afaaadea"
        ],
        "Size": 473960231,
        "SharedSize": 0,
        "VirtualSize": 473960231,
        "Labels": {
            "maintainer": "NVIDIA CORPORATION \u003ccudatools@nvidia.com\u003e",
            "org.opencontainers.image.created": "2025-10-07T11:36:49.254Z",
            "org.opencontainers.image.description": "A docker image for the pixi package manager 🐳 📦",
            "org.opencontainers.image.licenses": "BSD-3-Clause",
            "org.opencontainers.image.ref.name": "ubuntu",
            "org.opencontainers.image.revision": "9cf19c9fb7645bc73229d764104825fb854c5aa0",
            "org.opencontainers.image.source": "https://github.com/prefix-dev/pixi-docker",
            "org.opencontainers.image.title": "pixi-docker",
            "org.opencontainers.image.url": "https://github.com/prefix-dev/pixi-docker",
            "org.opencontainers.image.version": "noble-cuda-13.0.0"
        },
        "Containers": 0,
        "Digest": "sha256:d2b1b423bd60adb9ce8b739ba065409ac0024611fe8439944111a5c7afaaadea",
        "History": [
            "ghcr.io/prefix-dev/pixi:0.56.0-noble-cuda-13.0.0"
        ],
        "Names": [
            "ghcr.io/prefix-dev/pixi:0.56.0-noble-cuda-13.0.0"
        ],
        "Created": 1759837033,
        "CreatedAt": "2025-10-07T11:37:13Z"
    }
]