Move Docker base image to Debian trixie for Java 21

Planetiler 0.10.2 needs Java 21+; bookworm's default-jre-headless (and
its backports) only goes up to 17, causing a real build to fail with
UnsupportedClassVersionError. trixie's default-jre-headless is Java 21.

Dockerfile

@@ -1,4 +1,7 @@
-FROM ruby:4.0.6-slim-bookworm
+# trixie, not bookworm: Planetiler needs Java 21+ (UnsupportedClassVersionError
+# otherwise) and bookworm's own default-jre-headless is only Java 17 —
+# bookworm-backports doesn't carry a newer one either, only trixie's does.
+FROM ruby:4.0.6-slim-trixie
 
 # build-essential: puma has a native (C) extension; harmless if the
 # platform-specific gem in Gemfile.lock already has a precompiled binary
@@ -6,7 +9,8 @@ FROM ruby:4.0.6-slim-bookworm
 # ca-certificates/curl: needed for the go-pmtiles/planetiler.jar downloads
 # below (also handy for anyone debugging inside a running container).
 # default-jre-headless: Planetiler is a JVM tool (see PLANETILER_VERSION
-# below) — headless since this never touches a display.
+# below) — headless since this never touches a display; trixie's version
+# of this package is Java 21, see above.
 # osmium-tool: merges multiple countries' OSM extracts into one file before
 # handing them to Planetiler (lib/maptool/pmtiles_builder.rb) — a normal
 # Debian package, no separate download/pinning needed like the other two.