Fix traefik behavior when network_mode is host

[FuNK3Y]

What does this PR do?

When traefik cannot identify the ip address of the container, it returns 127.0.0.1 no matter what. This behavior is not desirable in many situations (i.e. exposed container running in network_mode = host while traefik doesn't, docker desktop, ...)

Motivation

It solves #5559 and #5535 and maybe a few other I didn't catch

Additional Notes

For this to work in linux, moby/moby#40007 needs to be merged. In the meantime you need to define "host.docker.internal" using extra_hosts if you want to take advantage of it.

In any case, this won't alter the current behavior. If you are running traefik in network_mode host or on a platform/version that does not provide the dns record its behavior won't change.

[FuNK3Y]

Anything I can do to speed the merging of this one?

[FuNK3Y]

@ldez the underlying PR (moby/moby#40007) just got merged. Any idea when this one will get reviewed?

[juliens]

@FuNK3Y WDYT about my reword on the documentation?

[FuNK3Y]

Looks good. You might keep the docker desktop part ouf of it; in my initial PR I mentioned it, but it turns out that host mode is not supported:
The host networking driver only works on Linux hosts, and is not supported on Docker Desktop for Mac, Docker Desktop for Windows, or Docker EE for Windows Server. (https://docs.docker.com/network/host/)

[juliens]

LGTM

[ldez]

Thanks 👍

[bjeanes]

It's worth noting that this broke a unit test which is still broken on master, but possibly only on hosts with host.docker.internal resolveable.

--- FAIL: TestDockerGetIPAddress (0.00s)
    --- FAIL: TestDockerGetIPAddress/two_networks,_no_network_label,_mode_host (0.00s)
        config_test.go:3256:
                Error Trace:    config_test.go:3256
                Error:          Not equal:
                                expected: "127.0.0.1"
                                actual  : "10.144.231.49"

                                Diff:
                                --- Expected
                                +++ Actual
                                @@ -1 +1 @@
                                -127.0.0.1
                                +10.144.231.49
                Test:           TestDockerGetIPAddress/two_networks,_no_network_label,_mode_host
    --- FAIL: TestDockerGetIPAddress/no_network,_no_network_label,_mode_host (0.00s)
        config_test.go:3256:
                Error Trace:    config_test.go:3256
                Error:          Not equal:
                                expected: "127.0.0.1"
                                actual  : "10.144.231.49"

                                Diff:
                                --- Expected
                                +++ Actual
                                @@ -1 +1 @@
                                -127.0.0.1
                                +10.144.231.49
                Test:           TestDockerGetIPAddress/no_network,_no_network_label,_mode_host