Support Ubuntu Core

### Summary

Cannot currently make this work on Ubuntu Core within a strictly confined snap

### Description

I've been trying to get ecs anywhere running on Ubuntu Core for a few days and have managed it.
Is there any appetite for accepting a merge request?

### Expected Behavior

Should be able to run on Ubuntu Core

### Observed Behavior

* Many paths mounted into the ecs-agent docker container do not exist outside of the strictly confined snaps context
* Some of the iptables directories mounted into the ecs-agent docker container do not exist and can't be created due to a read-only filesystem

### Environment Details

Ubuntu Core 20.04

### Supporting Log Snippets

Start by reviewing the Ubuntu Core 20.04 strict-confinement limitations described here, especially the missing host paths and unavailable or read-only iptables directories mounted into the ecs-agent Docker container. Done means ECS Anywhere can run on Ubuntu Core within a strictly confined snap, but the issue does not name implementation files or tests.

Written by the indexing model from the issue text.