docker: pin build-env base to node:22-bookworm

The CI deploy runs inside the prebuilt changkun/modern-cpp-tutorial:build-env
image. Per the Docker Hub API that image was last pushed 2019-07-15 and
never updated -- built from node:latest, which was Node 12 at the time.
Hexo 8 requires Node >= 20.19, so that stale image cannot run the
upgraded toolchain; pushing the Hexo 8 upgrade without rebuilding it will
break the deploy at 'hexo generate'.

Pin to a concrete current LTS so the rebuild is deterministic and won't
silently drift again. NOTE: this Dockerfile change is untested -- the
image still dpkg-installs a 2019 pandoc 2.7.3 .deb and texlive-full, and
CI's make target builds pdf+epub as well as the site, so a rebuild needs
a one-off test ('make build-env' then a full 'make') before relying on
it to deploy.

Author: changkun

docker/Dockerfile

@@ -1,4 +1,7 @@
-FROM node:latest
+# Pin to a concrete, current Node LTS. The toolchain (Hexo 8) requires
+# Node >= 20.19; an unpinned `node:latest` silently drifts, and the build-env
+# image was last rebuilt in 2020 (Node 15), which Hexo 8 will not run on.
+FROM node:22-bookworm
 
 LABEL "maintainer"="Changkun Ou <hi[at]changkun.de>"
 LABEL "repository"="https://github.com/changkun/modern-cpp-tutorial"