Run yarn build during docker build (#1158)

This will alliviate the merge conflicts we keep getting by checking this
file into source control.


-----------

Now, when using docker-compose, to re-generate the `styles.css` file,
you can do so locally in the following ways:

1. locally run `yarn build`
2. run `docker compose build`

.gitignore

@@ -156,6 +156,9 @@ python.log
 # mailman
 mailman/web/static/
 
+# static files, rendered by docker.
+static/css/styles.css
+
 # dev
 *.code-workspace
 .vscode/settings.json