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 |

.github/workflows/test.yml

@@ -1,8 +1,7 @@
+# DigitalOcean E2E tests (manual trigger only)
+# Primary CI testing is now done via Docker in docker-test.yml
+# This workflow is kept for real-world VM testing when needed
 on:
-  push:
-    branches:
-      - master
-      - ci
   workflow_dispatch:
 
 name: Test