Upgrade base image from Debian Bullseye (oldstable) to Bookworm (stable)

Avail ourselves of newer (though still not _new_ ;) software, in
particular the SQLite CLI.

The use of Bullseye's version number, 11, instead of its name in the
builder-build-platform image tripped me up at first, so I've switched it
over to Bookworm's name instead of number (12).

The cross-compilation of vcftools targeting arm64/aarch64 on amd64
requires pkg-config installed for the target platform with xx-apt-get.
Without it, the vcftool's configure script can't find zlib since its
using the build platform's pkg-config and zlib is not installed for the
build platform.  I'm not sure why this worked previously but not now.

Dockerfile

@@ -13,7 +13,7 @@ FROM --platform=$BUILDPLATFORM tonistiigi/xx AS xx
 # Define a builder stage that runs on the build platform.
 # Even if the target platform is different, instructions will run natively for
 # faster compilation.
-FROM --platform=$BUILDPLATFORM debian:11-slim AS builder-build-platform
+FROM --platform=$BUILDPLATFORM debian:bookworm-slim AS builder-build-platform
 
 SHELL ["/bin/bash", "-e", "-u", "-o", "pipefail", "-c"]
 
@@ -63,6 +63,7 @@ RUN xx-apt-get install -y \
   gcc \
   g++ \
   libc6-dev \
+  pkg-config \
   zlib1g-dev
 
 # Add dependencies. All should be pinned to specific versions, except
@@ -224,7 +225,7 @@ RUN curl -fsSL https://ftp.ncbi.nlm.nih.gov/pub/datasets/command-line/v2/linux-$
 # This is in place for Python programs which are not easy to install for a
 # different target platform¹.
 # ¹ https://github.com/pypa/pip/issues/5453
-FROM --platform=$TARGETPLATFORM python:3.10-slim-bullseye AS builder-target-platform
+FROM --platform=$TARGETPLATFORM python:3.10-slim-bookworm AS builder-target-platform
 
 SHELL ["/bin/bash", "-e", "-u", "-o", "pipefail", "-c"]
 
@@ -350,7 +351,7 @@ RUN pip3 install evofr
 # ———————————————————————————————————————————————————————————————————— #
 
 # Now build the final image.
-FROM python:3.10-slim-bullseye AS final
+FROM python:3.10-slim-bookworm AS final
 
 SHELL ["/bin/bash", "-e", "-u", "-o", "pipefail", "-c"]