update docker instructions in README (#3502)

you need the fancy hugos.

README.md

@@ -63,13 +63,15 @@ place after an MSC has been accepted, not as part of a proposal itself.
    <https://gohugo.io/getting-started/installing>. Note that at least Hugo
    v0.74 is required.
 
-   Alternatively, use the Docker image at https://hub.docker.com/r/klakegg/hugo/.
+   Alternatively, use the Docker image at
+   https://hub.docker.com/r/klakegg/hugo/. (The "extended edition" is required
+   to process the SCSS.)
 2. Run `git submodule update --init --recursive` for good measure.
 3. Run `npm i` to install the dependencies. Note that this will require NodeJS to be installed.
 4. Run `npm run get-proposals` to seed proposal data. This is merely for populating the content of the "Spec Change Proposals"
    page and is not required.
 5. Run `hugo serve` (or `docker run --rm -it -v $(pwd):/src -p 1313:1313
-   klakegg/hugo serve`) to run a local webserver which builds whenever a file
+   klakegg/hugo:ext serve`) to run a local webserver which builds whenever a file
    change is detected. If watching doesn't appear to be working for you, try
    adding `--disableFastRender` to the commandline.
 6. Edit the specification 🙂