feat: Enable local docker ip in for communication with outside k3d

[devops-42]

Motivation

This PR introduces the possibility to connect to services outside the k3d cluster used in DevContainer by setting up a distinct hostname.

Modifications

The magic hostname host.docker.internal is re-used and injected in the devcontainer.json and the CoreDNS configmap.

Verification

Within the same Docker environment a Web container running nginx has been created. After that, VSCode with DevContainer is started. When DevContainer is up and running, exec a shell in a pod and issue:

kubectl exec -ti deploy/minio -- curl -v telnet://host.docker.internal:80
* Rebuilt URL to: telnet://host.docker.internal:80/
*   Trying 172.17.0.1...
* TCP_NODELAY set
* Connected to host.docker.internal (172.17.0.1) port 80 (#0)

[terrytangyuan]

@Joibel @tczhao Could you help review this?

[terrytangyuan]

@devops-42 Please sign-off your commits.

[devops-42]

@terrytangyuan Commits are signed off now.

[terrytangyuan]

Thanks. Let's wait for others to review.

[Joibel]

The code looks sane enough, but I think there should be some documentation added to docs/running-locally.md to explain that this is possible and why you might want to use it.

I suppose I'm not sure of the use case that this really helps with that just running the same thing inside k3d doesn't. Documentation is needed anyway, but explaining when to use it may help me understand it too.

[devops-42]

@Joibel Thanks for your comments! I have added some documentation which hopefully should clarify the use-case.

[Joibel]

LGTM

[Joibel]

@devops-42 thank you for contributing.