add runtime stage in Dockerfile

Dockerfile

-FROM rust:1.73 AS builder
+FROM rust:1.76 AS runtime
 
 RUN cargo install mdbook mdbook-toc mdbook-mermaid
 
+FROM runtime as builder
+
 WORKDIR /opt/src
 COPY . .
 RUN mdbook build
 
 FROM nginx:latest
 
-COPY --from=builder /opt/src/book/html /usr/share/nginx/html
+COPY --from=builder /opt/src/book/html /usr/share/nginx/html
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