Update the Docker image base and Rust builder

Author: Marek Suchánek

Dockerfile

@@ -11,13 +11,13 @@
 # # CMD ["newdoc"]
 # ENTRYPOINT ["newdoc"]
 
-## This configuration results in a small image based on Debian Buster without Rust tooling
-FROM rust:1.47 as builder
+## This configuration results in a small image based on Debian Bullseye without Rust tooling
+FROM rust:1.50 as builder
 WORKDIR /usr/src/newdoc
 COPY . .
 RUN cargo install --path .
 
-FROM debian:buster-slim
+FROM debian:bullseye-slim
 # RUN apt-get update && apt-get install -y extra-runtime-dependencies
 COPY --from=builder /usr/local/cargo/bin/newdoc /usr/local/bin/newdoc
 ENTRYPOINT ["newdoc"]