Add Docker-based E2E testing (#1320)

### Summary
- Add automated end-to-end testing using Docker to verify the installation script works across 18 Linux distributions
- Add Oracle Linux 9 support to the installation script
- Drop support for EOL distributions (Debian 8/9/10, CentOS 7, Ubuntu 16.04) 
- Disable Digital Ocean droplets based end-to-end tests, let's use docker from now on

### Changes
**New test infrastructure:**
- `test/Dockerfile.server` - Multi-OS server image with `BASE_IMAGE` build arg
- `test/Dockerfile.client` - Ubuntu 24.04 client for connectivity testing
- `test/server-entrypoint.sh` - Runs install script, verifies files exist, asserts iptables NAT rules, starts OpenVPN
- `test/client-entrypoint.sh` - Connects to VPN, verifies tun0 interface, pings gateway
- `docker-compose.yml` - Orchestrates server + client with shared volume
- `.github/workflows/docker-test.yml` - CI matrix testing 18 OS variants
- `.github/workflows/test.yml` - Removed push/PR triggers, now manual only for DO tests
- `Makefile` - Local testing commands (`make test`, `make test-ubuntu-24.04`, etc.)

**Distributions tested (18 total):**
| Family | Versions |
|--------|----------|
| Ubuntu | 18.04, 20.04, 22.04, 24.04 |
| Debian | 11, 12 |
| Fedora | 40, 41 |
| Rocky Linux | 8, 9 |
| AlmaLinux | 8, 9 |
| Oracle Linux | 8, 9 |
| Amazon Linux | 2, 2023 |
| CentOS Stream | 9 |
| Arch Linux | latest |

Makefile

@@ -0,0 +1,136 @@
+.PHONY: test test-build test-up test-down test-logs test-clean
+
+# Run the full test suite
+test: test-build test-up
+	@echo "Waiting for tests to complete..."
+	@for i in $$(seq 1 60); do \
+		if docker logs openvpn-client 2>&1 | grep -q "ALL TESTS PASSED"; then \
+			echo "✓ Tests passed!"; \
+			$(MAKE) test-down; \
+			exit 0; \
+		fi; \
+		if docker logs openvpn-client 2>&1 | grep -q "FAIL:"; then \
+			echo "✗ Tests failed!"; \
+			docker logs openvpn-client; \
+			$(MAKE) test-down; \
+			exit 1; \
+		fi; \
+		echo "Waiting... ($$i/60)"; \
+		sleep 2; \
+	done; \
+	echo "Timeout waiting for tests"; \
+	$(MAKE) test-down; \
+	exit 1
+
+# Build test containers
+test-build:
+	BASE_IMAGE=$(BASE_IMAGE) docker compose build
+
+# Start test containers
+test-up:
+	docker compose up -d
+
+# Stop and remove test containers
+test-down:
+	docker compose down -v --remove-orphans
+
+# View logs
+test-logs:
+	docker compose logs -f
+
+# View server logs only
+test-logs-server:
+	docker logs -f openvpn-server
+
+# View client logs only
+test-logs-client:
+	docker logs -f openvpn-client
+
+# Full cleanup
+test-clean: test-down
+	docker rmi openvpn-install-openvpn-server openvpn-install-openvpn-client 2>/dev/null || true
+	docker volume prune -f
+
+# Interactive shell into server container
+test-shell-server:
+	docker exec -it openvpn-server /bin/bash
+
+# Interactive shell into client container
+test-shell-client:
+	docker exec -it openvpn-client /bin/bash
+
+# Test specific distributions
+test-ubuntu-18.04:
+	$(MAKE) test BASE_IMAGE=ubuntu:18.04
+
+test-ubuntu-20.04:
+	$(MAKE) test BASE_IMAGE=ubuntu:20.04
+
+test-ubuntu-22.04:
+	$(MAKE) test BASE_IMAGE=ubuntu:22.04
+
+test-ubuntu-24.04:
+	$(MAKE) test BASE_IMAGE=ubuntu:24.04
+
+test-debian-11:
+	$(MAKE) test BASE_IMAGE=debian:11
+
+test-debian-12:
+	$(MAKE) test BASE_IMAGE=debian:12
+
+test-fedora-40:
+	$(MAKE) test BASE_IMAGE=fedora:40
+
+test-fedora-41:
+	$(MAKE) test BASE_IMAGE=fedora:41
+
+test-rocky-8:
+	$(MAKE) test BASE_IMAGE=rockylinux:8
+
+test-rocky-9:
+	$(MAKE) test BASE_IMAGE=rockylinux:9
+
+test-almalinux-8:
+	$(MAKE) test BASE_IMAGE=almalinux:8
+
+test-almalinux-9:
+	$(MAKE) test BASE_IMAGE=almalinux:9
+
+test-oracle-8:
+	$(MAKE) test BASE_IMAGE=oraclelinux:8
+
+test-oracle-9:
+	$(MAKE) test BASE_IMAGE=oraclelinux:9
+
+test-amazon-2:
+	$(MAKE) test BASE_IMAGE=amazonlinux:2
+
+test-amazon-2023:
+	$(MAKE) test BASE_IMAGE=amazonlinux:2023
+
+test-arch:
+	$(MAKE) test BASE_IMAGE=archlinux:latest
+
+test-centos-stream-9:
+	$(MAKE) test BASE_IMAGE=quay.io/centos/centos:stream9
+
+# Test all distributions (runs sequentially)
+test-all:
+	$(MAKE) test-ubuntu-18.04
+	$(MAKE) test-ubuntu-20.04
+	$(MAKE) test-ubuntu-22.04
+	$(MAKE) test-ubuntu-24.04
+	$(MAKE) test-debian-11
+	$(MAKE) test-debian-12
+	$(MAKE) test-fedora-40
+	$(MAKE) test-fedora-41
+	$(MAKE) test-rocky-8
+	$(MAKE) test-rocky-9
+	$(MAKE) test-almalinux-8
+	$(MAKE) test-almalinux-9
+	$(MAKE) test-oracle-8
+	$(MAKE) test-oracle-9
+	$(MAKE) test-amazon-2
+	$(MAKE) test-amazon-2023
+	$(MAKE) test-arch
+	$(MAKE) test-centos-stream-9