Add container support for Docker Swarm environments

### Description

Currently, Glance uses the local docker socket to gather container information on the system it's running on. In a docker swarm environment, containers are distributed across multiple hosts (nodes) as docker services. As a result, Glance does not display all labeled docker containers running in the swarm, only the containers currently running on the same node as it is.

The glance agent could be used and deployed globally in the swarm too send container information to the Glance app. This can work the same way services like Portainer and Swarmpit gather container information using their agents. From [Portainer's GitHub](https://github.com/portainer/agent) page:

> The Portainer agent is basically a cluster of Docker API proxies. Deployed inside a Swarm cluster on each node, it allows the redirection (proxy) of a Docker API request on any specific node as well as the aggregration of the response of multiple nodes.

I see no potential downsides as swarm mode can be enabled via an environmental variable which defaults to false. Anyone outside a docker swarm environment can simply omit any swarm related configurations.