docs: add docker builder prune to pruning page

Fixes #17590. Adds a new "Prune build cache" section covering
docker builder prune, including the --all flag for removing all
build cache (not just dangling entries).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: dvdksn

content/manuals/engine/manage-resources/pruning.md

@@ -1,11 +1,12 @@
 ---
 description: Free up disk space by removing unused resources with the prune command
-keywords: pruning, prune, images, volumes, containers, networks, disk, administration,
+keywords:
+  pruning, prune, images, volumes, containers, networks, disk, administration,
   garbage collection
 title: Prune unused Docker objects
 aliases:
-- /engine/admin/pruning/
-- /config/pruning/
+  - /engine/admin/pruning/
+  - /config/pruning/
 ---
 
 Docker takes a conservative approach to cleaning up unused objects (often
@@ -143,6 +144,35 @@ Other filtering expressions are available. See the
 [`docker network prune` reference](/reference/cli/docker/network/prune/)
 for more examples.
 
+## Prune build cache
+
+The build cache is used by the BuildKit builder to speed up subsequent builds.
+Over time, this cache can grow large. Use `docker builder prune` to remove
+dangling build cache — cache that isn't associated with any current build:
+
+```console
+$ docker builder prune
+
+WARNING! This will remove all dangling build cache.
+Are you sure you want to continue? [y/N] y
+```
+
+By default, you're prompted to continue. To bypass the prompt, use the `-f` or
+`--force` flag.
+
+To remove all build cache, not just dangling cache, use the `--all` flag:
+
+```console
+$ docker builder prune --all
+
+WARNING! This will remove all build cache.
+Are you sure you want to continue? [y/N] y
+```
+
+Other filtering expressions are available. See the
+[`docker builder prune` reference](/reference/cli/docker/builder/prune/)
+for more examples.
+
 ## Prune everything
 
 The `docker system prune` command is a shortcut that prunes images, containers,