`NextjsGlobalFunctions`: image optimization returns 400 for imported images

Hi, thanks for this great construct — it made deploying Next.js 16 on AWS with CDK really smooth. I ran into one issue with image optimization and wanted to report it.

## Summary

With `NextjsGlobalFunctions`, `next/image` optimization returns **400 (`The requested resource isn't a valid image ... received null`)** for imported images, which resolve to `/_next/static/media/*`:

```tsx
import Image from "next/image";
import foo from "./foo.png"; // -> /_next/static/media/foo..png

; // /_next/image request returns 400
```

Images referenced from `public/` by string path optimize fine:

```tsx
// works
```

## Environment

- `cdk-nextjs`: 0.5.12
- Next.js: 16.2 (App Router)
- Construct: `NextjsGlobalFunctions`

## Likely cause

The generated `functions.Dockerfile` copies `public/` into the container ("Next.js needs source images") but does not seem to copy `.next/static`:

```dockerfile
COPY --chown=nextjs:nodejs .next/standalone ./
COPY --chown=nextjs:nodejs public ./$RELATIVE_PATH_TO_PACKAGE/public
# no COPY of .next/static
```

`.next/static` (including `_next/static/media/*`) appears to be deployed only to the static-assets S3 bucket, not into the compute container. This might be why the optimizer can't read imported images as source (`received null`), while `public/` images work because `public/` is present in the container. I may be missing something here — happy to be corrected.

## Expected

Imported images (`_next/static/media/*`) should be optimizable, the same as `public/` images.

## Workaround

Placing images in `public/` and referencing them by string path works, since `public/` is copied into the container.

Happy to provide more details or logs if helpful. Thanks!

Start by inspecting the generated functions.Dockerfile and how .next/static is deployed relative to the static-assets S3 bucket and the compute container. Reproduce the next/image request with an imported image under _next/static/media, then verify that it optimizes successfully without the 400 response while public/ images continue to work.

Written by the indexing model from the issue text.