feat(ci): add multi-platform CI testing via Docker

Add CI infrastructure that tests the Ansible role on every distro
declared in meta/main.yml (EL 8/9, Debian bullseye/bookworm,
Ubuntu focal/jammy/noble) using Docker containers.

- ci/build_matrix.py: parse meta/main.yml platforms into JSON matrix
- ci/test_playbook.yml: test playbook (state=present + validate)
- ci/Dockerfile.el, ci/Dockerfile.debian: per-family Docker images
- Makefile: orchestrator (make test, make test-<slug>, make lint)
- .gitea/workflows/ci.yml: Gitea Actions with dynamic matrix
- .gitlab-ci.yml: GitLab CI pipeline
- Jenkinsfile: Jenkins pipeline with parallel stages
- .yamllint.yml: linter configuration
- .dockerignore: exclude .git and CI configs from Docker context

.gitlab-ci.yml

@@ -0,0 +1,24 @@
+---
+stages:
+  - lint
+  - test
+
+lint:
+  stage: lint
+  image: python:3.11-slim
+  before_script:
+    - pip install ansible-lint yamllint pyyaml
+  script:
+    - make lint
+
+test:
+  stage: test
+  image: docker:latest
+  services:
+    - docker:dind
+  variables:
+    DOCKER_TLS_CERTDIR: ""
+  before_script:
+    - apk add --no-cache python3 py3-pip py3-yaml make bash
+  script:
+    - make test