docs: add private registry documentation

Signed-off-by: Tomas Kral <tkral@redhat.com>
Assisted-by: Claude Code

Author: kadel

README.md

@@ -118,6 +118,28 @@ To load dynamic plugins from your local machine:
     - If no override file is present, `configs/dynamic-plugins/dynamic-plugins.yaml` will be used.
 4. See [Changing Your Configuration](#changing-your-configuration) for more on updating and reloading configs.
 
+## Accessing Private Container Registries
+
+If you need to install dynamic plugins from private container registries, you can mount your local container credentials into the `install-dynamic-plugins` service. This allows the plugin installer to authenticate with private registries using the credentials you already have configured on your system.
+
+Add the following volume to the `install-dynamic-plugins` service in `compose.yaml`:
+
+```yaml
+install-dynamic-plugins:
+  volumes:
+    # ... existing volumes ...
+    - ~/.config/containers/auth.json:/opt/app-root/src/.config/containers/auth.json:Z
+```
+
+This mounts your local `auth.json` file (which contains credentials for container registries like `registry.redhat.io`, `quay.io`, etc.) into the container. The credentials are typically created when you run `podman login` or `docker login`.
+
+> **NOTE**: The location of `auth.json` may vary depending on your system configuration. Common locations include:
+>
+> - `~/.config/containers/auth.json` (Podman default)
+> - `~/.docker/config.json` (Docker default)
+>
+> Adjust the source path accordingly based on your container runtime.
+
 ## Changing The Container Image
 
 You can switch between RHDH and Janus-IDP by changing the container image name hold by the `RHDH_IMAGE` environment variable in your `.env` file.