ci: update runner-image + repo path to public/ namespace

Repos moved to public/ on Gitea: public/runner-image (CI image)
and public/code-crispies (this repo). Updates workflow image refs,
spec hyperlinks, README, and compose tags.

.gitea/workflows/ci.yml

@@ -10,7 +10,7 @@ jobs:
   ci:
     runs-on: ubuntu-latest
     container:
-      image: git.librete.ch/libretech/runner-image:v1
+      image: git.librete.ch/public/runner-image:v1
     timeout-minutes: 10
     steps:
       - uses: actions/checkout@v4