engine/storage/containerd: Document lazy pulling

With the nydus, overlaybd, soci, or stargz snapshotter, docker pull
skips the content of layers that the snapshotter provides. If that
content isn't already stored locally, docker save fails for the image,
and docker push can fail too.

Add a "Lazy pulling" section that explains this default, which layers
it skips, and its limits. It notes that the default needs no
configuration, and that setting the lazy-pull feature overrides it.

Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>

_vale/config/vocabularies/Docker/accept.txt

@@ -211,6 +211,7 @@ Sonoma
 Splunk
 SQLite
 stargz
+eStargz
 stdin
 stdout
 subfolder

content/manuals/engine/storage/containerd.md

@@ -113,6 +113,48 @@ Docker Engine uses the `overlayfs` containerd snapshotter by default.
 > image store, push them to a registry first, or use `docker save` to export
 > them.
 
+## Lazy pulling
+
+With lazy pulling, containers can start before their images are fully downloaded.
+Remote snapshotters fetch layer data from the registry as containers need it.
+
+Docker Engine enables lazy pulling by default when configured to use the `nydus`, `overlaybd`, `soci`, or `stargz` snapshotter.
+With other snapshotters, including the default `overlayfs`, Docker downloads all layer content before the container starts unless you explicitly enable lazy pulling.
+
+During a lazy pull, Docker skips downloading a layer if the snapshotter can mount it remotely or already has its snapshot.
+Docker downloads all other layers.
+For example, the stargz snapshotter can mount eStargz layers remotely, but plain gzip layers must be downloaded unless their snapshots already exist.
+
+### Saving and pushing images
+
+Lazily pulled images can run even if some layer content is missing from the containerd content store.
+However, missing content affects saving and pushing:
+
+- `docker save` can fail or produce an incomplete archive without reporting an error.
+- `docker push` cannot upload missing layers, but can reuse them if the target registry supports mounting them from the source repository.
+
+Running the image is not guaranteed to fill in the missing content in the containerd content store.
+For example, the stargz snapshotter keeps the data it fetches in its own cache.
+See [moby#53878](https://github.com/moby/moby/issues/53878) for details.
+
+### Configuration
+
+To disable lazy-pulling, set the `lazy-pull` feature to `false` in `/etc/docker/daemon.json`:
+
+```json
+{
+  "features": {
+    "containerd-snapshotter": true,
+    "lazy-pull": false
+  }
+}
+```
+
+Reload the daemon configuration to apply the setting to subsequent pulls.
+For images you already pulled lazily, pull them again to download the missing content.
+
+To enable lazy pulling for a snapshotter outside the default list, set `lazy-pull` to `true` instead.
+
 ## Experimental automatic migration
 
 Docker Engine includes an experimental feature that can automatically switch to