The Dockerfile asked cargo for --features python-config, which cannot
select the litellm-ai-gateway bin target: that target carries
required-features = ["server"], so cargo silently built nothing and the
later COPY of /build/litellm-rust/target/release/litellm-ai-gateway had
no file to copy. Turn the server feature on and name the bin explicitly
so a future required-features drift fails at the cargo step instead of
silently producing an empty release dir.