Add offline/air-gapped deployment support for REST API and MCP Workbench

[dustins]

Here's a polished version of your description:

---

Description

This PR adds support for offline and air-gapped deployments by enabling configuration of pre-cached external dependencies for both the REST API container and MCP Workbench via custom-config.yaml.

Changes

REST API Container - Nodeenv Pre-caching

Added support for pre-caching nodeenv (required by prisma-client-py) in the REST API container. The implementation maintains backward compatibility while enabling offline deployments through a configurable cache directory path.

# Configure offline build dependencies for REST API (nodeenv for prisma-client-py)
restApiConfig:
  buildConfig:
    NODEENV_CACHE_DIR: "./nodeenv-cache"  # Path relative to lib/serve/rest-api/

• Backward Compatible: Default behavior unchanged (downloads nodeenv at runtime build)
• Offline Support: Supports pre-cached nodeenv for environments without internet access

MCP Workbench - Dependency Source Override

Added support for overriding dependency sources for S6 Overlay and rclone used in MCP Workbench to support offline/air-gapped deployments through configurable source paths.

# Configure offline build dependencies for MCP Workbench (S6 Overlay and rclone)
mcpWorkbenchBuildConfig:
  S6_OVERLAY_NOARCH_SOURCE: "./s6-overlay-noarch.tar.xz"  # Path relative to lib/serve/mcp-workbench/
  S6_OVERLAY_ARCH_SOURCE: "./s6-overlay-x86_64.tar.xz"    # Path relative to lib/serve/mcp-workbench/
  RCLONE_SOURCE: "./rclone-linux-amd64.zip"                # Path relative to lib/serve/mcp-workbench/

• Backward Compatible: Default behavior unchanged (downloads dependencies during build)
• Consistent Pattern: All offline build configurations follow the same approach

Testing

• Verified default behavior remains unchanged
• Tested offline deployment with pre-cached dependencies
• Validated configuration options in custom-config.yaml