CI: generate archlinux-webengine-qt6 docker image

I'm not 100% sure we need this. But there is an existing test matrix
that I want to active for now. We can review all the configuration later
I guess?

.github/workflows/docker.yml

@@ -15,6 +15,7 @@ jobs:
           - archlinux-webkit
           - archlinux-webengine
           - archlinux-webengine-unstable
+          - archlinux-webengine-qt6
     steps:
       - uses: actions/checkout@v3
       - uses: actions/setup-python@v4