support static IP address in network config?

[laixintao]

Hi can we support static IP address?

Why do I want it:
I want to have etcd running in a container provided by footloose, but there are some configs don't support domain, like this one:
https://etcd.io/docs/v3.4/op-guide/configuration/#--listen-peer-urls . So I can't write some thing like http://etcd0:2379 in config, instead I have to put an IP address.

Docker support assigning static IP address (https://docs.docker.com/engine/reference/commandline/run/#connect-a-container-to-a-network---network) (with --network), I am wondering can we support this in footloose's config?

If you are ok with this feature, I'd like to submit a PR. :)