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>

action.yml

@@ -61,6 +61,11 @@ inputs:
       Docker volume mounts. Default to /tmp:/tmp
     default: /tmp:/tmp
     required: false
+  docker-ssh:
+    description: |
+      List of SSH agent socket or keys to expose to the build.
+      This maps to the --ssh flag in docker buildx build.
+    required: false
 
 runs:
   using: node20