build(docker): granite is the standalone stack's default embedding model

settings.EMBEDDINGS_NAME still defaults to mpnet so an upgraded deployment
keeps its index; new installs only get granite because setup.sh writes it to
.env. The standalone compose always starts on fresh volumes, so it defaults
EMBEDDINGS_NAME to granite (overridable from .env or the shell), and the
Docker guide's hand-written .env examples name the model too.

docs/content/Deploying/Docker-Deploying.mdx

@@ -42,7 +42,9 @@ first use. You do not need the source tree to run them:
 
     `INTERNAL_KEY` is the secret the worker uses to hand finished indexes to
     the API; without it every upload fails with a 401. `setup.sh` generates
-    one for you, a hand-written `.env` has to include it.
+    one for you, a hand-written `.env` has to include it. This stack runs the
+    granite embedding model unless `.env` sets `EMBEDDINGS_NAME`; both granite
+    and mpnet are baked into the image.
 
 3.  **Start it:**
 
@@ -91,9 +93,13 @@ extra packages: `EXTRAS=docling` in `.env`).
     LLM_PROVIDER=docsgpt
     VITE_API_STREAMING=true
     INTERNAL_KEY=<any random string, e.g. openssl rand -hex 16>
+    EMBEDDINGS_NAME=ibm-granite/granite-embedding-311m-multilingual-r2
     ```
 
-    This minimal configuration tells DocsGPT to use the public API. For more advanced settings and other LLM options, refer to the [DocsGPT Settings Guide](/Deploying/DocsGPT-Settings).
+    This minimal configuration tells DocsGPT to use the public API. The
+    `EMBEDDINGS_NAME` line is what `setup.sh` writes for a new install; without
+    it the code falls back to mpnet, the model earlier releases indexed with,
+    so that an upgraded deployment keeps its existing index working. For more advanced settings and other LLM options, refer to the [DocsGPT Settings Guide](/Deploying/DocsGPT-Settings).
 
 4.  **Launch DocsGPT with Docker Compose:**