Set gateway Pod IP as GatewayStatus address

[sjberman]

To satisfy conformance and provide the address of the gateway, we'll now set the pod IP address of the gateway on the GatewayStatus resource. Eventually we will also support other addresses.

Closes #370

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

[sjberman]

@pleshakov I've added the env var to the entrypoint and performed validation. Let me know what you think. I originally was going to mimic the existing validation pattern for arguments, but I changed my mind. For one, I wouldn't expect that we would be supported many env vars, so may not be worth it. Also, if we did want to do it, there's likely a way to refactor the existing argument validation to combine args and env vars, but I figured that's probably a bit too much work for right now, and went with the simpler approach. If we feel that it's worth implementing now though, I can do that.

[pleshakov]

@sjberman

@pleshakov I originally was going to mimic the existing validation pattern for arguments, but I changed my mind. For one, I wouldn't expect that we would be supported many env vars, so may not be worth it. Also, if we did want to do it, there's likely a way to refactor the existing argument validation to combine args and env vars, but I figured that's probably a bit too much work for right now, and went with the simpler approach. If we feel that it's worth implementing now though, I can do that.

This makes sense to me. However, it made me think - do we need to have an env variable at all?

what if we add --external-ips cli arg that will accept the list of IPs (a list, because the Gateway status can include many IPs). By default, we can initialize it to the POD_IP. For example:

        args:
        - --gateway-ctlr-name=k8s-gateway.nginx.org/nginx-gateway-controller
        - --gatewayclass=nginx
        - --external-ips=$(POD_IP)

Could that work better?

[sjberman]

it made me think - do we need to have an env variable at all

We do need it; in order to use the k8s downward API to get the pod IP, it has to be an environment variable. I'm also not a huge fan of more CLI args if they can be avoided.