JSON update-check report output

[mordilloSan]

Feature Request Checklist

• I checked existing issues and discussions for duplicates.
• This is a feature request, not a support question.

Problem Description

I would like to consume Watchtower update-check results from another service/dashboard.

Today --porcelain v1 provides stable machine-readable text, which is useful, but consumers still need to parse Watchtower-specific text output to answer questions like:

• which containers were checked?
• which containers have an update available?
• which containers are fresh?
• which containers failed or were skipped?
• what image was checked?

I only need a stable JSON representation of the update report.

Desired Solution

Would you be open to a PR that adds stable JSON report output for update checks?

Possible shapes, depending on what you would prefer:

1. Add a JSON porcelain format, for example:

watchtower --run-once --monitor-only --porcelain json

Or add a new porcelain version, for example:

watchtower --run-once --monitor-only --porcelain v2

Or expose the most recent / current update report through the HTTP API, for example:

GET /v1/status

The JSON would ideally include per-container results, using Watchtower’s existing result/report data where possible:
{
"containers": [
{
"name": "nginx",
"image": "nginx:latest",
"state": "stale",
"update_available": true,
"error": ""
}
]
}

The exact schema is flexible. I mainly want to ask whether this kind of feature would be acceptable before working on a PR.

Current Alternatives or Workarounds

Parse --porcelain v1 text output externally.
Use notifications/templates and parse the notification payload.
Use the HTTP API summary response, but it does not provide enough per-container update availability detail for dashboard/status use cases.
Use Prometheus metrics, but those are aggregate and do not expose per-container update availability.

Additional Context

--porcelain json seems the cleanest change.`