enhance(k8s): reach Kubernetes services by their id when it is a DNS subdomain (#1320)

On Kubernetes a service is reached by its id made a DNS label, so some service ids might become unreachable. For example, the `devstoreaccount1.azurite.local` service of Gitea's [MSSQL tests](https://github.com/go-gitea/gitea/blob/2fe74d3cdd5184a7e22e32e47772caa78d077489/.github/workflows/pull-db-tests.yml#L229) is only in the pod's hosts as `devstoreaccount1-azurite-local`, while the tests connect to the id as written. Container names must be DNS labels, but `hostAliases` accepts DNS subdomains, so an id that is one is now added there too, and to `no_proxy`, as on Docker where a service is reached by its id.

The warning for a service not reachable by its id now also covers ids that differ only in case, as Kubernetes hostnames are lowercase.

---------

Co-authored-by: silverwind <[email protected]>
Reviewed-on: https://gitea.com/gitea/runner/pulls/1320
Reviewed-by: Lunny Xiao <[email protected]>
Reviewed-by: bircni <[email protected]>
Reviewed-by: silverwind <[email protected]>
Co-authored-by: Zettat123 <[email protected]>

docs/kubernetes.md

@@ -51,7 +51,7 @@ Job pods mount no service account token unless the template sets `automountServi
 
 ## Services
 
-Services share the pod's network, so a step reaches one by its id or `localhost`, and two services cannot listen on the same port. An id is lowercased and has characters other than letters, digits and `-` replaced by `-`, so the service `my_db` is reached as `my-db`. Pass secrets to services through `env`, as `command` is visible in the pod spec. Of `options`, only `--health-*` is supported. They become a startup probe with Docker's defaults, which stops a service that fails its retries, so the job fails as on Docker.
+Services share the pod's network, so a step reaches one by its id or `localhost`, and two services cannot listen on the same port. An id that is not a valid hostname is sanitized, so `my_db` is reached as `my-db`. Pass secrets to services through `env`, as `command` is visible in the pod spec. Of `options`, only `--health-*` is supported. They become a startup probe with Docker's defaults, which stops a service that fails its retries, so the job fails as on Docker.
 
 ## Unsupported