• Minimal (~11MB) Dockerfile container build.
• Available as mrnetops/fastly-exporter from Docker Hub.
• Set default namespace & subsystem (fastly and rt respectively).
• Read token from environment variable (FASTLY_API_TOKEN).
• Docker Compose for a full fastly-exporter + Prometheus +
  Grafana + Fastly dashboard stack.
• Fix a few typos in metric help messages.