fix: stop printing the image inspect JSON on destroy

[tas50]

image_exists? is the only predicate in the driver that shells out to docker without silencing it. Its three siblings -- container_exists?, container_running?, and image_in_use? -- all pass suppress_output.

So kitchen destroy on an instance with remove_images: true prints the image's entire docker inspect output -- config, environment, every layer digest, metadata, descriptor; about sixty lines of JSON -- in the middle of the destroy output, between the container being removed and the image being removed.

Nothing reads it. Only whether the command succeeded is used.

Before

-----> Destroying <default-ubuntu-2404>...
       [Docker] Destroying Docker container 282c247814fc40dc81b689df0168ec83ee536257f8004ed116257b401688b58d
       282c247814fc40dc81b689df0168ec83ee536257f8004ed116257b401688b58d
       282c247814fc40dc81b689df0168ec83ee536257f8004ed116257b401688b58d
       [
           {
        "Id": "sha256:33c8e9c76cc4d4c1342411aded4f03c30fc69b964074505a1c0f3433c07b84a7",
        "RepoTags": [],
        "RepoDigests": [],
        "Comment": "buildkit.dockerfile.v0",
        "Created": "2026-08-23T16:32:50.994001424Z",
        "Config": { ... },
        "RootFS": {
            "Type": "layers",
            "Layers": [
                "sha256:646eea22414270d74b0c9e9d6d3b9550701ae62e658a099825d4d15045a3630b",
                ... 16 more ...
            ]
        },
        ...
           }
       ]
       [Docker] Removing image with Image ID sha256:33c8e9c76cc4...
       Deleted: sha256:33c8e9c76cc4...
       Finished destroying <default-ubuntu-2404> (0m0.29s).

After

-----> Destroying <default-ubuntu-2404>...
       [Docker] Destroying Docker container a2ad8e06fe96b8b7515b87f330e8677f5712adb896418fe53cdcda1ab5cfd496
       a2ad8e06fe96b8b7515b87f330e8677f5712adb896418fe53cdcda1ab5cfd496
       a2ad8e06fe96b8b7515b87f330e8677f5712adb896418fe53cdcda1ab5cfd496
       [Docker] Removing image with Image ID sha256:a659812d1948c2baaede157ea045cd61bae082cc48964bc82605b0062d895197.
       Deleted: sha256:a659812d1948c2baaede157ea045cd61bae082cc48964bc82605b0062d895197
       Finished destroying <default-ubuntu-2404> (0m0.31s).

Still printed under -l debug, where the rest of the driver's docker traffic goes -- confirmed in the same run.

The bare ids docker stop and docker rm echo are deliberately left alone: unlike this, they are what those commands report, not a predicate's incidental output.

The method also picked up an early return for the no-image case, so the guard reads the same way as container_exists? and image_in_use? rather than relying on a modifier rescue spanning an &&.

Confirmation

Docker 29.7.2 (Docker Desktop 4.87.0, macOS/arm64), Test Kitchen 4.1.1. Output above is from real kitchen create / kitchen destroy runs against an ubuntu-24.04 container with remove_images: true.

rake style clean; rspec 318 examples, 0 failures (5 new).