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 |

test/Dockerfile.client

@@ -0,0 +1,24 @@
+# checkov:skip=CKV_DOCKER_2:Test container doesn't need healthcheck
+# checkov:skip=CKV_DOCKER_3:OpenVPN client requires root for NET_ADMIN
+FROM ubuntu:24.04
+
+ENV DEBIAN_FRONTEND=noninteractive
+
+# Install OpenVPN client and testing tools
+RUN apt-get update && apt-get install -y \
+    openvpn \
+    iproute2 \
+    iputils-ping \
+    procps \
+    && rm -rf /var/lib/apt/lists/*
+
+# Create TUN device directory (device will be mounted at runtime)
+RUN mkdir -p /dev/net
+
+# Copy test scripts
+COPY test/client-entrypoint.sh /entrypoint.sh
+RUN chmod +x /entrypoint.sh
+
+WORKDIR /etc/openvpn
+
+ENTRYPOINT ["/entrypoint.sh"]