fix: Ensure OpenRC is installed on Alpine Linux systems

What does this MR do?

Implements Alpine Linux packaging support by automatically installing OpenRC as a dependency when gitlab-runner is installed on Alpine Linux systems. This resolves the rc-service: executable file not found in $PATH error that occurs when the kardianos/service library attempts to manage the gitlab-runner service.

Why was this MR needed?

Fixes #39210 (closed). The gitlab-runner binary fails on Alpine Linux systems because OpenRC (which provides the rc-service command) is not included in Alpine's minimal base image. The kardianos/service library requires rc-service for service management, but users installing gitlab-runner on bare Alpine systems had to manually install OpenRC first.

This MR provides a clean, packaging-level solution that automatically ensures OpenRC is available when the gitlab-runner package is installed.

What's the best way to test this MR?

Validate the OpenRC installation logic without building packages:

# 1. Verify script syntax
sh -n packaging/scripts/postinst.apk

# 2. Test OpenRC installation in Alpine container
docker run --rm --privileged alpine:3.18 sh -c '
  # Configure repositories (use HTTP to avoid local SSL issues)
  echo "http://dl-cdn.alpinelinux.org/alpine/v3.18/main" > /etc/apk/repositories
  echo "http://dl-cdn.alpinelinux.org/alpine/v3.18/community" >> /etc/apk/repositories
  
  # Verify initial state
  echo "=== Initial State ==="
  command -v rc-service || echo "rc-service not found (expected)"
  
  # Run postinst.apk OpenRC installation logic
  echo -e "\n=== Installing OpenRC ==="
  if ! command -v rc-service >/dev/null 2>&1; then
    echo "Installing OpenRC as a dependency for gitlab-runner..."
    apk add --no-cache openrc
  fi
  
  # Verify success
  echo -e "\n=== Verification ==="
  command -v rc-service && echo "SUCCESS: rc-service is available"
  rc-service --help | head -3
'

Expected output: OpenRC installs successfully and rc-service command becomes available.

What are the relevant issue numbers?

Closes #39210 (closed)