add Docker and CI/CD pipeline with ledo

Configure Next.js static export with nginx serving, Dockerfile
(multi-stage build), docker-compose for prod/dev, Gitea Actions
pipelines for build-on-push and tag-based releases using ledo tool.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.ledo.yml

@@ -0,0 +1,11 @@
+runtime: docker
+docker:
+  registry: git.cynarski.pl
+  namespace: automancer
+  name: site
+  main_service: site
+  shell: /bin/bash
+modes:
+  base: docker/docker-compose.yml
+  dev: docker/docker-compose.yml docker/docker-compose.dev.yml
+  test: docker/docker-compose.yml docker/docker-compose.test.yml