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

Makefile

@@ -0,0 +1,57 @@
+.PHONY: help matrix lint test clean
+
+SHELL := /bin/bash
+IMAGE_PREFIX := remote-users-fact-test
+
+help: ## Show this help
+	@grep -E '^[a-zA-Z_-]+:.*##' $(MAKEFILE_LIST) | awk 'BEGIN {FS = ":.*##"}; {printf "  \033[36m%-20s\033[0m %s\n", $$1, $$2}'
+
+matrix: ## Print the test matrix as JSON
+	@python3 ci/build_matrix.py
+
+lint: ## Run yamllint and ansible-lint
+	yamllint roles/
+	ansible-lint
+
+test: ## Run tests on all distros from the matrix
+	@python3 ci/build_matrix.py | python3 -c "\
+	import sys, json; \
+	matrix = json.load(sys.stdin); \
+	[print(e['slug'] + ' ' + e['image'] + ' ' + e['platform']) for e in matrix]" | \
+	while read slug image platform; do \
+		$(MAKE) _test-one SLUG=$$slug IMAGE=$$image PLATFORM=$$platform; \
+	done
+
+test-%: ## Run test on a single distro (e.g. make test-el9)
+	@python3 ci/build_matrix.py | python3 -c "\
+	import sys, json; \
+	slug = '$(*)';\
+	matrix = json.load(sys.stdin); \
+	matches = [e for e in matrix if e['slug'] == slug]; \
+	assert matches, f'Unknown slug: {slug}. Valid: {[e[\"slug\"] for e in matrix]}'; \
+	e = matches[0]; print(e['slug'] + ' ' + e['image'] + ' ' + e['platform'])" | \
+	while read slug image platform; do \
+		$(MAKE) _test-one SLUG=$$slug IMAGE=$$image PLATFORM=$$platform; \
+	done
+
+_test-one:
+	@echo "=== Testing $(SLUG) ($(IMAGE)) ==="
+	@dockerfile=ci/Dockerfile.el; \
+	if [ "$(PLATFORM)" = "Debian" ] || [ "$(PLATFORM)" = "Ubuntu" ]; then \
+		dockerfile=ci/Dockerfile.debian; \
+	fi; \
+	docker build \
+		--build-arg "IMAGE=$(IMAGE)" \
+		-t "$(IMAGE_PREFIX)-$(SLUG)" \
+		-f "$$dockerfile" . \
+	&& docker run --rm "$(IMAGE_PREFIX)-$(SLUG)"
+	@echo "=== $(SLUG) OK ==="
+
+clean: ## Remove test Docker images
+	@python3 ci/build_matrix.py | python3 -c "\
+	import sys, json; \
+	[print('$(IMAGE_PREFIX)-' + e['slug']) for e in json.load(sys.stdin)]" | \
+	while read img; do \
+		docker rmi $$img 2>/dev/null || true; \
+	done
+	@echo "Clean done."