docs: spell out what the local log driver actually changes

The log-rotation section asserted that json-file "grows unbounded" without
saying why or what local replaces it with, which is the kind of claim a reader
has to go verify before acting on it.

Per Docker's driver docs: json-file defaults to max-size -1 (unlimited) with
max-file 1, so it does not rotate at all unless configured; local defaults to
20 MB x 5 files with compression on, capping a container at roughly 100 MB.
Adds that comparison as a table, notes docker logs still works, and records the
caveat that local's files are "designed to be exclusively accessed by the Docker
daemon".

Also stops overclaiming: Docker's docs state no preference between the two
drivers, so this is presented as a better-defaults choice, with the json-file +
log-opts alternative given for anyone who needs raw-JSON compatibility. Notes
that the setting only affects newly created containers, which makes a post-reset
moment the cheapest time to apply it.

Author: gatezh

docs/docker-maintenance-cheatsheet.md

@@ -157,8 +157,28 @@ recreates the disk and destroys all volumes — back up first.
 }
 ```
 
-`local` rotates and compresses; the default `json-file` grows unbounded.
-Requires a Docker restart, and applies to newly created containers.
+Every line a container writes to stdout/stderr is stored on disk, and the default
+`json-file` driver **never rotates unless you tell it to** — its `max-size`
+defaults to `-1` (unlimited) with `max-file: 1`. A chatty dev server or a
+database logging every query grows that file forever, and it shows up nowhere in
+`docker system df`.
+
+| | `json-file` (default) | `local` |
+|---|---|---|
+| Rotation | none (`max-size: -1`, `max-file: 1`) | 20 MB × 5 files |
+| Compression | no | yes, by default |
+| Cap per container | unbounded | ~100 MB |
+
+`docker logs` works normally with `local`. Docker's docs do *not* state a
+preference between the two drivers, so this is a better-defaults choice rather
+than an official recommendation — the equally valid alternative is keeping
+`json-file` and adding `"log-opts": {"max-size": "10m", "max-file": "3"}`, which
+preserves compatibility if anything ever parses the raw JSON log files. With
+`local`, don't: Docker warns those files "are designed to be exclusively
+accessed by the Docker daemon".
+
+Requires a Docker restart, and applies only to **newly created** containers — so
+the cheapest moment to change it is right after a reset, when none exist.
 
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