Add support for update via the web interface (for RPi OS devices).

[nicomiguelino]

Overview

• Since the web interface is running inside a Docker container, we need to execute an update command outside of the containers to perform an update. (Thanks to @vpetersson for giving insights.)
• We can use the Redis subscriber component running inside the host RPi OS. That component listens from incoming messages.
• From the web server container, we can send a message.
• We just need to modify host_agent.py to handle the update.