Add support for github enterprise private instance endpoint

[mtrienis]

I think the github client defaults to using github.com for an endpoint, is it possible to add support for a private github enterprise endpoint?

[mtrienis]

Maybe something like this:

--- ci/github/github.go
+++ ci/github/github.go
@@ -3,6 +3,7 @@
 import (
 	"context"
 	"fmt"
 	"log/slog"
 	"os"
 	"strconv"
 	"strings"
 
 	"github.com/diggerhq/digger/libs/ci"
 	"github.com/diggerhq/digger/libs/ci/generic"
 	"github.com/diggerhq/digger/libs/scheduler"
 
 	"github.com/diggerhq/digger/libs/digger_config"
 	"github.com/dominikbraun/graph"
 
 	"github.com/google/go-github/v61/github"
 )
 
 type GithubServiceProvider interface {
 	NewService(ghToken string, repoName string, owner string) (GithubService, error)
 }
 
 type GithubServiceProviderBasic struct{}
 
 func (_ GithubServiceProviderBasic) NewService(ghToken string, repoName string, owner string) (GithubService, error) {
-	client := github.NewClient(nil)
+	var client *github.Client
+	var err error
+
+	// Check for Enterprise URL in standard GHA environment variable
+	enterpriseUrl := os.Getenv("GITHUB_API_URL")
+	if enterpriseUrl != "" && enterpriseUrl != "https://api.github.com" {
+		// GHE requires base URL and upload URL (usually the same)
+		client, err = github.NewEnterpriseClient(enterpriseUrl, enterpriseUrl, nil)
+		if err != nil {
+			return GithubService{}, fmt.Errorf("failed to create enterprise client: %v", err)
+		}
+	} else {
+		client = github.NewClient(nil)
+	}
+
 	if ghToken != "" {
 		client = client.WithAuthToken(ghToken)
 	}
 
 	return GithubService{
 		Client:   client,
 		RepoName: repoName,
 		Owner:    owner,
 	}, nil
 }
 
 type GithubService struct {
 	Client   *github.Client
 	RepoName string
 	Owner    string
 }