ci: add version accumulation to Helm index with merge

Author: JeromeBu

.github/workflows/ci.yaml

@@ -133,9 +133,9 @@ jobs:
           gh release download v${TO_VERSION} --pattern "catalogi-*.tgz" --dir docs/charts/
         env:
           GH_TOKEN: ${{ github.token }}
-      - name: Generate Helm repository index
+      - name: Generate Helm repository index with merge
         run: |
-          helm repo index docs/charts/ --url https://github.com/codegouvfr/catalogi/releases/download/v${TO_VERSION}/
+          helm repo index docs/charts/ --url https://github.com/codegouvfr/catalogi/releases/download/v${TO_VERSION}/ --merge docs/charts/index.yaml
       - name: Commit and push index.yaml
         run: |
           git config --local user.email "actions@github.com"