Add support for Podman

Description

Right now the monitoring node only works with Docker. It should be possible to run with Podman.

This may help later on when providing container images.

Tasks

• Make the container runtime configurable through the .env file.
• Update the maintenance scripts to support both runtimes.
• Abstract the container handling with an onionprobe-monitor script.
• Update the documentation.
• Test the monitoring node with Podman.

Time estimation

• Complexity: small (1 day)
• Uncertainty: low (x1.1)
• Reference (adapted)