better-sqlite3@12.11.1 ships prebuilt binaries for ABI 127/137/141/147 only;
Node 20 is ABI 115, so prebuild-install falls back to node-gyp, which fails on
node:20-slim for lack of python3/make/g++.
Add the toolchain to the builder stage, and in the runtime stage install, use
and purge it inside a single layer so the final image ships no compiler.
