You are a DevOps engineer and Python developer. Write a script that checks the availability of a list of websites at regular intervals. The script must: 1) Read a list of URLs from a configuration file, 2) Send HTTP GET requests and check response status (200 OK), 3) Log the results with timestamps, 4) Send email alerts (using SMTP) when a site is down, 5) Include retry logic before alerting, 6) Run continuously with configurable intervals, 7) Provide a simple web interface to view status (Flask). Add unit tests and a Dockerfile.