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 |

README.md

@@ -132,26 +132,23 @@ export PASS="1"
 
 The script supports these Linux distributions:
 
-|                        | Support |
-| ---------------------- | ------- |
-| AlmaLinux 8            | ✅      |
-| Amazon Linux 2         | ✅      |
-| Amazon Linux >= 2023.6 | ✅      |
-| Arch Linux             | ✅      |
-| CentOS 7               | ✅      |
-| CentOS Stream >= 8     | ✅ 🤖   |
-| Debian >= 10           | ✅ 🤖   |
-| Fedora >= 35           | ✅ 🤖   |
-| Oracle Linux 8         | ✅      |
-| Rocky Linux 8          | ✅      |
-| Ubuntu >= 18.04        | ✅ 🤖   |
+|                    | Support |
+| ------------------ | ------- |
+| AlmaLinux >= 8     | ✅ 🤖   |
+| Amazon Linux 2     | ✅ 🤖   |
+| Amazon Linux 2023  | ✅ 🤖   |
+| Arch Linux         | ✅ 🤖   |
+| CentOS Stream >= 8 | ✅ 🤖   |
+| Debian >= 11       | ✅ 🤖   |
+| Fedora >= 40       | ✅ 🤖   |
+| Oracle Linux >= 8  | ✅ 🤖   |
+| Rocky Linux >= 8   | ✅ 🤖   |
+| Ubuntu >= 18.04    | ✅ 🤖   |
 
 To be noted:
 
 - The script is regularly tested against the distributions marked with a 🤖 only.
   - It's only tested on `amd64` architecture.
-- It should work on older versions such as Debian 8+, Ubuntu 16.04+ and previous Fedora releases. But versions not in the table above are not officially supported.
-  - It should also support versions between the LTS versions, but these are not tested.
 - The script requires `systemd`.
 
 ## Fork