Build and run the backend on bookworm; bullseye's packages are no longer fetchable (#6)

Debian 11 left LTS at the end of August 2026. The runtime stage's
`apt-get install openssl` now fails: the index still names
openssl 1.1.1w-0+deb11u8 but deb.debian.org answers 404 for the file, so the
image cannot be built at all.

Moving only the runtime to bookworm would break the binaries: openssl-sys is in
the lock, so they link the system libssl, and a bullseye build wants libssl1.1,
which bookworm does not ship. Both stages move together: the builder becomes
rust:1.88-bookworm (the same toolchain ci-unified carried) and the runtime
bookworm-slim, so the binaries link the libssl3 they run with.

backend/Dockerfile

@@ -1,4 +1,4 @@
-FROM docker.io/paritytech/ci-unified:bullseye-1.88.0-2025-06-27-v202507221446 AS builder
+FROM docker.io/library/rust:1.88-bookworm AS builder
 
 ARG PROFILE=release
 WORKDIR /app
@@ -8,7 +8,7 @@ COPY . .
 RUN cargo build --${PROFILE} --bins -j 2
 
 # MAIN IMAGE FOR PEOPLE TO PULL --- small one#
-FROM docker.io/debian:bullseye-slim
+FROM docker.io/debian:bookworm-slim
 LABEL maintainer="Dijital Kurdistan Tech Institute <admin@pezkuwichain.io>"
 LABEL description="Pezkuwichain Telemetry Backend shard/core binaries"