Use host-gateway to ensure servers can reach host

[DMRobertson]

This fixes connectivity problems I was having when running complement. Oddly, noone else seems to have found this issue, and I don't really understand the underlying machinations.

I'm not sure if this will cause complement to fail in environments whose docker versions are older than 20.10.

[erikjohnston]

I'm not sure if this will cause complement to fail in environments whose docker versions are older than 20.10.

Don't suppose there is an easy way to test the docker version?

[DMRobertson]

Don't suppose there is an easy way to test the docker version?

Either:

• ask if anyone has docker < 20.10 installed, or
• see if I can downgrade, or
• docker-in-docker? https://hub.docker.com/_/docker

[DMRobertson]

Don't suppose there is an easy way to test the docker version?

Erik means: can we interrogate the docker version at runtime?

It looks like this is exposed as docker.dockerversion.Version. Presumably this is some string that needs to be parsed. https://pkg.go.dev/github.com/docker/docker@v20.10.17+incompatible/dockerversion

[kegsay]

Please add docs to the README to say you need Docker 20+, then LGTM