Upgrade docker base image to 3.14-rc-slim, in chapter 9

Author: hjwp

chapter_09_docker.asciidoc

@@ -496,7 +496,7 @@ next to the _src/_ directory we made earlier:
 ====
 [source,dockerfile]
 ----
-FROM python:3.14-slim  # <1>
+FROM python:3.14-rc-slim  # <1>
 
 COPY src /src  # <2>
 
@@ -517,6 +517,8 @@ CMD ["python", "manage.py", "runserver"]  # <4>
     called "slim" because it's as small as possible.
     It's based on a popular version of Linux called Debian,
     and of course it comes with Python already installed on it.
+    By the time you're reading this book, 3.14 should be past the release candidate stage,
+    so you should use `3.14-slim`.
 
 <2> The `COPY` instruction (the uppercase words are called "instructions")
     lets you copy files from your own computer into the container image.