sveltekit nodejs adapter: Allow passing config.kit.paths.assets dynamically from environment variable

Describe the problem

Hi,

We're building SvelteKit app into a Docker image using SvelteKit Node.js adapter.

We'd like to use a single Docker image in all environments but set a different CDN URL (config.kit.paths.assets) based on where it runs (dev/staging/production; different production AWS regions around the World).

/** @type {import('@sveltejs/kit').Config} */
const config = {
	preprocess: preprocess(),
	kit: {
		paths: {
			assets: "\${process.env.CDN_URL}",
		},
		adapter: node({
			out: 'build',
			precompress: true,
			envPrefix: '',
		}),
	},
};

When we tried to set config.kit.paths.assets to a dynamic value like this, the build failed on validation

Error: config.kit.paths.assets option must be an absolute path, if specified. See https://kit.svelte.dev/docs/configuration#paths

Describe the proposed solution

We'd like to relax config.kit.paths.assets validation (https://kit.svelte.dev/docs/configuration#paths) to allows dynamic values like "${process.env.CDN_URL}", that can be resolved in the runtime by Node.

We'd like to pass this CDN_URL environment variable to the Docker (Kubernetes) container in the runtime based on where it runs.

Alternatives considered

1. A global CDN URL. This is not something our customer allows.
2. Run sed replacement during the container boot time. This might work but feels too brittle.

Importance

would make my life easier

Additional Information

No response

Start with the SvelteKit Node.js adapter and the validation for config.kit.paths.assets described in the issue. Trace how the value is handled during the build and at runtime, then verify the built Docker/Kubernetes application can receive CDN_URL at runtime without rejecting the dynamic value during validation.