fix: index.html and marketing assets not served in Docker deployment

[Adityatorgal17]

Bug Description

When running Maglev via Docker, GET / returns 404 index.html not found
instead of the welcome page.

Root Cause

index.html and marketing/ are copied into the builder stage via COPY . ., but the runtime stage
(based on alpine:3.21) only explicitly copies the binary and
config.example.json. Static web assets are never transferred to the final
image.

Additionally, marketing/ was listed in .dockerignore which would have
prevented it from reaching the builder stage in a fresh build context.

Steps to Reproduce

1. Clone the repo
2. cp config.docker.example.json config.docker.json
3. docker compose up
4. curl http://localhost:4000/

Expected: Welcome page HTML (status 200)
Actual: index.html not found (status 404)

Proposed Fix

• Add COPY --from=builder /build/index.html ./index.html to the runtime stage in Dockerfile
• Add COPY --from=builder /build/marketing/ ./marketing/ to the runtime stage in Dockerfile
• Remove marketing/ from .dockerignore so it is available in the builder stage

I have a fix ready and will open a PR referencing this issue.