Ensure Docker builds include any CSS needed when the CMS Admin mode i…

…s active, too

To test this, I did `make build prod` to build a new release image, locally,
then shelled in:

$ pwd
/app
$ ls static/admin/css
autocomplete.4a81fc4242d0.css  dark_mode.ef27a31af300.css  nav_sidebar.269a1bd44627.css     rtl.css
autocomplete.css               dashboard.css               nav_sidebar.css                  vendor
base.523eb49842a7.css          dashboard.e90f2068217b.css  responsive.css                   widgets.css
base.css                       forms.c14e1cb06392.css      responsive.f6533dab034d.css      widgets.ee33ab26c7c2.css
changelists.9237a1ac391b.css   forms.css                   responsive_rtl.7d1130848605.css
changelists.css                login.586129c60a93.css      responsive_rtl.css
dark_mode.css                  login.css                   rtl.512d4b53fc59.css

I also compared this with a release image made yesterday, before this change:

$ pwd
/app
$ ls static/admin/
ls: cannot access 'static/admin/': No such file or directory

docker/bin/build_staticfiles.sh

@@ -9,8 +9,8 @@ set -exo pipefail
 rm -rf ./static
 
 if [[ "$1" == "--nolink" ]]; then
-    python manage.py collectstatic --noinput -v 0
+    WAGTAIL_ENABLE_ADMIN=True python manage.py collectstatic --noinput -v 0
 else
-    python manage.py collectstatic -l --noinput -v 0
+    WAGTAIL_ENABLE_ADMIN=True python manage.py collectstatic -l --noinput -v 0
     docker/bin/softlinkstatic.py
 fi