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

README.md

@@ -66,6 +66,10 @@
 
 Visit `localhost:8080` in web browser
 
+### start a shell
+
+    $ make shell
+
 ### start watcher
 
     $ docker exec -it container_name sitegen watch
@@ -90,6 +94,13 @@ example:
 
     $ make deploy
 
+### renew server TSL certs
+
+SSH into server admin account and run:
+
+    $ cd www
+    $ dehydrated --config config -c
+
 ## gotchas
 
 ### What is error `cosmo failed: [string "..."]:62: cannot resume dead coroutine`?