Docker: Properly set the FEATURES ENV in the 2nd build stage

See: moby/moby#37345

Fixes #48

Dockerfile

@@ -6,6 +6,7 @@ ARG FEATURES="electrum http webhooks track-spends"
 RUN cargo install --locked --path . --no-default-features --features "$FEATURES"
 
 FROM debian:buster-slim
+ARG FEATURES="electrum http webhooks track-spends"
 RUN echo $FEATURES | grep -v webhooks > /dev/null || (apt-get update && apt-get install -y libssl-dev)
 COPY --from=builder /usr/local/cargo/bin/bwt /usr/local/bin/
 ENTRYPOINT [ "bwt", "--bitcoind-dir", "/bitcoin" ]