PublishWithStaticFiles and PublishAsStaticWebsite should allow Dockerfile customization

## Is there an existing issue for this?

- [x] I have searched the existing issues

## Describe the bug

Neither `YarpResourceExtensions.PublishWithStaticFiles` nor the new `JavaScriptHostingExtensions.PublishAsStaticWebsite` (from #15736) provide any way to customize the generated Dockerfile content.

### YARP `PublishWithStaticFiles`

Introduced in #12483. Internally calls `WithDockerfileBuilder` via the private `EnsurePublishWithStaticFilesDockerFileBuilder()` method, which generates a fixed Dockerfile (FROM yarp → COPY to /app/wwwroot). No configure callback or overload exists to modify this.

### JavaScript `PublishAsStaticWebsite`

Introduced in #15736. Has an `Action` configure callback, but that only controls `OutputPath`, `StripPrefix`, and `TargetEndpointName`. There's no way to customize the actual Dockerfile — it delegates to the YARP layer which has the same limitation.

## Expected Behavior

Both methods should provide a way to customize the generated Dockerfile content — either via:
- A `Func` callback parameter
- An overload that accepts additional Dockerfile stages/instructions
- A configure callback that exposes the `DockerfileBuilder`

This is especially important since `WithDockerfileBuilder` is not available in polyglot AppHosts (see #15858), so TS/Python/Java users have no workaround at all.

## Use Cases

- Adding custom COPY instructions (e.g., additional config files, .env files)
- Adding RUN instructions (e.g., installing extra packages in the container)
- Customizing the base image beyond what `WithDockerfileBaseImage` provides
- Adding multi-stage build steps before the final YARP stage

## Anything else?

Related: #15858 (WithDockerfileBuilder not available in polyglot AppHosts)