fix(ci): evaluate short SHA in bash step and include flux-system in root kustomization (#47)

### Summary of Changes

- Evaluated `short_sha` inside a bash step (`id: vars`) so Docker
`build-args` receives the expanded short SHA (e.g. `sha-4429a65`) rather
than literal bash code strings.
- Added `- flux-system` to
`ci/k8s/clusters/production/kustomization.yaml` to prevent Flux from
pruning its own `GitRepository` resource during reconciliation.

.github/workflows/services-docs-next-required.yml

@@ -81,6 +81,10 @@ jobs:
             type=raw,value=latest
             type=sha,format=short
 
+      - name: Compute short SHA
+        id: vars
+        run: echo "short_sha=sha-$(echo ${{ github.sha }} | cut -c1-7)" >> $GITHUB_OUTPUT
+
       - name: Build and push Docker image
         uses: docker/build-push-action@v5
         with:
@@ -90,8 +94,8 @@ jobs:
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}
           build-args: |
-            COMMIT_SHA=sha-$(echo ${{ github.sha }} | cut -c1-7)
-            VERSION=sha-$(echo ${{ github.sha }} | cut -c1-7)
+            COMMIT_SHA=${{ steps.vars.outputs.short_sha }}
+            VERSION=${{ steps.vars.outputs.short_sha }}
 
   deploy:
     name: Deploy - Kubernetes (GitOps)