Resolves GLIBC_2.39 mismatch error — CI builds on ubuntu-latest
(24.04) which links against glibc 2.39, but debian:bookworm-slim
only provides glibc 2.36, causing binary execution failure.
- Strip builder stage from all Rust Dockerfiles; images now only contain
the runtime and a pre-built binary copied from target/.
- build.js: cargo builds all Rust binaries first (using all CPU cores),
then copies them into Docker images.
- .drone.yml: add cargo-build step before docker-build so kaniko can COPY
the pre-compiled binaries without rebuilding inside the image.
