[Fix] Use default gateway, when bridge network doesn't have it (#666, @kuritka)

[kuritka]

What

If there is no default gateway in the bridge network, k3d crashes because it tries to parse the empty gateway string. This PR will use the default gateway if the gateway does not exist. A cluster can be created than.

Why

For some time docker does not create an automatic gateway in case I create a new network. When I create a bridge network without a gateway and try to connect it to k3d, the cluster installation of the latest versions of k3d crashes.

For example, this won't work for me:

docker network create my-network --subnet=172.18.0.0/16
k3d cluster create my-cluster --network=my-network

        "IPAM": {
            "Driver": "default",
            "Options": {},
            "Config": [
                {
                    "Subnet": "172.18.0.0/16"
                }
            ]
        },

Right now we have to do something like this manually:

docker network create my-gw-network --subnet=172.22.0.0/16 --gateway=172.22.0.1
k3d cluster create my-cluster --network=my-gw-network

        "IPAM": {
            "Driver": "default",
            "Options": {},
            "Config": [
                {
                    "Subnet": "172.22.0.0/16",
                    "Gateway": "172.22.0.1"
                }
            ]
        },

^^^ passing

Implications

our k3d action worked fine on k3d v4.4.1, until 28.6.2021, when the GitHub runner upgraded to Ubuntu Version: 20210628.1. This forced us to upgrade k3d-action a bit earlier than we had planned. Unfortunately, we found out that the newer implementation can't handle the missing gateway. This change does not affect the CLI or Docker settings. It only handles internally the case when the gateway is missing in docker network settings.

@all-contributors add @kuritka for code, maintenance

Signed-off-by: kuritka kuritka@gmail.com

[iwilltry42]

Hi @kuritka , thanks for this PR!
Changes in the GH Actions VM may also explain #663.
The addition makes total sense for me, thank you!

[iwilltry42]

LGTM