fix: Partially revert "chore: Remove support for legacy swarm"
This partially reverts commit 2494e207843c92a715da3e8e65ed763fd0d2d624
by ignoring any network named "ingress" when searching for a
container's IP address.
That commit was technically a backwards-incompatible change: Some
users use nginx-proxy with Swarm mode even though it is not fully
supported. In such cases nginx-proxy should ignore the `ingress`
network, otherwise nginx will not be able to reach the
server (container-to-container traffic apparently doesn't work over
the Swarm `ingress` network).
The parts of that commit that examine the `SwarmNode` structure are
not reverted here because docker-gen does not currently populate that
structure -- not even when both docker-gen and the service task
container are running on the same manager node.