Add containerisation strategy details and CI image build step

Expand architecture doc section 4.5 with image building process,
container registry management, and deployment pipeline prose.
Add Docker build & push to Gitea OCI registry in CI workflow.

Co-authored-by: Cursor <cursoragent@cursor.com>

.github/workflows/release.yaml

@@ -121,6 +121,17 @@ jobs:
           mv fleetdm-stack-*.tgz .tmp/
           ls -la .tmp/
 
+      - name: Build and push container image
+        run: |
+          TAG="${{ steps.version.outputs.new_tag }}"
+          IMAGE="git.produktor.io/${{ gitea.repository }}/fleetdm-stack"
+
+          echo "${{ gitea.token }}" | docker login git.produktor.io -u "${{ gitea.actor }}" --password-stdin
+          docker build -t "${IMAGE}:${TAG}" -t "${IMAGE}:latest" .
+          docker push "${IMAGE}:${TAG}"
+          docker push "${IMAGE}:latest"
+          echo "Pushed ${IMAGE}:${TAG}"
+
       - name: Create tag
         run: |
           git config user.name "Gitea Actions"