- build.js: cargo build first, then docker build — no more Docker-in-Docker
- .dockerignore: exclude target/ but re-include
target/x86_64-unknown-linux-gnu/release/ so pre-built binaries
are available in Docker build context
The .dockerignore excluded the entire target/ directory, preventing
COPY of pre-built binaries into runtime-only Docker images.
Now explicitly allows target/x86_64-unknown-linux-gnu/release/.