feat(helm): Add support for OCI chart repositories (#445)

Signed-off-by: João Fernandes <jcsf_1995@hotmail.com>

README.md

@@ -131,6 +131,7 @@ Notice that if no config file is specified, then `ct.yaml` (or any of the suppor
 #### Using private chart repositories
 
 When adding chart-repos you can specify additional arguments for the `helm repo add` command using `helm-repo-extra-args` on a per-repo basis.
+You can also specify OCI registries which will be added using the `helm registry login` command, they also support the `helm-repo-extra-args` for authentication.
 This could for example be used to authenticate a private chart repository.
 
 `config.yaml`:
@@ -140,6 +141,7 @@ chart-repos:
   - incubator=https://incubator.io
   - basic-auth=https://private.com
   - ssl-repo=https://self-signed.ca
+  - oci-registry=oci://nice-oci-registry.pt
 helm-repo-extra-args:
   - ssl-repo=--ca-file ./my-ca.crt
 ```

pkg/tool/helm.go

@@ -16,6 +16,7 @@ package tool
 
 import (
 	"fmt"
+	"strings"
 
 	"github.com/helm/chart-testing/v3/pkg/exec"
 )
@@ -35,6 +36,13 @@ func NewHelm(exec exec.ProcessExecutor, extraArgs []string, extraSetArgs []strin
 }
 
 func (h Helm) AddRepo(name string, url string, extraArgs []string) error {
+	const ociPrefix string = "oci://"
+
+	if strings.HasPrefix(url, ociPrefix) {
+		registryDomain := url[len(ociPrefix):]
+		return h.exec.RunProcess("helm", "registry", "login", registryDomain, extraArgs)
+	}
+
 	return h.exec.RunProcess("helm", "repo", "add", name, url, extraArgs)
 }