feat: add docker-ssh option to expose SSH credentials to container

Add a new `docker-ssh` input option that allows exposing SSH agent sockets
or keys to the Renovate Docker container via the `--ssh` flag. This enables
Renovate to access private repositories or registries that require SSH
authentication during the build process.

The implementation follows the same pattern as other docker-* options and
includes full documentation with usage examples.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

README.md

@@ -14,6 +14,7 @@ GitHub Action to run Renovate self-hosted.
   - [`docker-cmd-file`](#docker-cmd-file)
   - [`docker-network`](#docker-network)
   - [`docker-socket-host-path`](#docker-socket-host-path)
+  - [`docker-ssh`](#docker-ssh)
   - [`docker-user`](#docker-user)
   - [`docker-volumes`](#docker-volumes)
   - [`env-regex`](#env-regex)
@@ -120,6 +121,29 @@ Allows the overriding of the host path for the Docker socket that is mounted int
 Useful on systems where the host Docker socket is located somewhere other than `/var/run/docker.sock` (the default).
 Only applicable when `mount-docker-socket` is true.
 
+### `docker-ssh`
+
+List of SSH agent socket or keys to expose to the build. This is passed to the Docker container via the `--ssh` flag.
+
+This is useful when Renovate needs to access private repositories or registries via SSH during the build process.
+
+Example usage:
+
+```yml
+....
+jobs:
+  renovate:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4.2.2
+      - name: Self-hosted Renovate
+        uses: renovatebot/github-action@v43.0.17
+        with:
+          token: ${{ secrets.RENOVATE_TOKEN }}
+          docker-ssh: default=${{ env.SSH_AUTH_SOCK }}
+```
+
 ### `docker-user`
 
 Specify a user (or user-id) to run docker command.