Update base container image from "bookworm" to "trixie" (Debian 13) (#4)

These changes:

- Update base container image from Bookworm to Trixie Debian
- Update Pandoc version
- Add a `make shell` command

Caveats:

- Uses the "bookworm" version of OpenResty until "trixie" is released

Reviewed-on: #4

Makefile

@@ -5,6 +5,9 @@ serve:
 	docker run --rm -it --init -v $(PWD):/opt/app -p 8080:80 $(image) \
 		sh -c "openresty -p /opt/app -g 'daemon off;' -c conf/dev.nginx.conf"
 
+shell:
+	docker run --rm -it --init -v $(PWD):/opt/app -p 8080:80 $(image) bash
+
 build:
 	$(if $(file), $(docker-run) sitegen build $(file), $(docker-run) sitegen)