Add first-class JSON log output (Log4j2 JsonTemplateLayout) via env var

### Enhancement Type

Improve an existing feature

### Describe the enhancement

**Describe the problem / motivation**
Many log pipelines and docker observation tools work best with JSON in container stdout. Today the image primarily supports customizing PatternLayout via `LOG_CONSOLE_FORMAT` / `LOG_FILE_FORMAT`, which still produces unstructured text. ([docker-minecraft-server.readthedocs.io](https://docker-minecraft-server.readthedocs.io/en/latest/configuration/misc-options/))

**Proposed enhancement**
Add an opt-in way to switch console (and optionally file) logs to **JSON output** using Log4j2’s **JsonTemplateLayout**. ([logging.apache.org](https://logging.apache.org/log4j/2.x/manual/json-template-layout.html))

Suggested UX:

* `LOG_FORMAT=json` (or `LOG_LAYOUT=json-template`)
* `LOG_JSON_TEMPLATE=`

Start by locating the configuration that handles LOG_CONSOLE_FORMAT and LOG_FILE_FORMAT, then review Log4j2 JsonTemplateLayout and the linked documentation. Done means an opt-in environment variable produces JSON console logs, with the proposed optional file-log and template behavior clarified and covered by appropriate validation.

Written by the indexing model from the issue text.