Fix worker Docker build: pin builder to bookworm to match runtime glibc

rust:1.97 (unsuffixed) now resolves to a trixie base, whose newer glibc
produced a binary the bookworm-slim runtime stage couldn't load
("GLIBC_2.38 not found (required by mcmapper-worker)"), crash-looping the
container on every start. Pinning the builder to rust:1.97-bookworm matches
the runtime's Debian release. Confirmed by rebuilding and running the real
image — worker now starts and runs (its next failure is an unrelated,
expected MinIO connectivity gap, not this).

worker/Dockerfile

@@ -1,4 +1,7 @@
-FROM rust:1.97 AS build
+# Pinned to -bookworm to match the bookworm-slim runtime stage below — the plain `rust:1.97` tag
+# now resolves to a trixie base, whose newer glibc produced binaries the runtime's older glibc
+# couldn't load ("GLIBC_2.38 not found"), confirmed by actually running the built image.
+FROM rust:1.97-bookworm AS build
 WORKDIR /app
 COPY . .
 RUN cargo build --release