docker: don't keep source or toolchain in container image

Dockerfile

@@ -1,8 +1,12 @@
-FROM rust:1.85
+FROM rust:1 AS builder
 
 WORKDIR /usr/src/mqtt-exporter
-COPY . .
+COPY src/ ./src
+COPY Cargo.* .
 
-RUN cargo install --path .
+RUN cargo install --path . --root /app
+
+FROM debian:bookworm-slim
+COPY --from=builder /app/bin/mqtt-exporter /usr/local/bin/mqtt-exporter
 
 CMD ["mqtt-exporter"]