koji: use the host name from /etc/redhat-release in CGImport metadata

As suggested by Brew maintainers Tomáš Kopeček and Lubomír Sedlář.

cmd/osbuild-worker/main.go

@@ -19,6 +19,7 @@ import (
 	"github.com/google/uuid"
 
 	"github.com/osbuild/osbuild-composer/internal/common"
+	"github.com/osbuild/osbuild-composer/internal/distro"
 	"github.com/osbuild/osbuild-composer/internal/osbuild"
 	"github.com/osbuild/osbuild-composer/internal/target"
 	"github.com/osbuild/osbuild-composer/internal/upload/awsupload"
@@ -236,6 +237,12 @@ func RunJob(job worker.Job, store string, kojiServers map[string]koji.GSSAPICred
 				continue
 			}
 
+			hostOS, err := distro.GetRedHatRelease()
+			if err != nil {
+				r = append(r, err)
+				continue
+			}
+
 			build := koji.ImageBuild{
 				BuildID:   options.BuildID,
 				TaskID:    options.TaskID,
@@ -249,7 +256,7 @@ func RunJob(job worker.Job, store string, kojiServers map[string]koji.GSSAPICred
 				{
 					ID: 1,
 					Host: koji.Host{
-						Os:   "RHEL8",
+						Os:   hostOS,
 						Arch: "noarch",
 					},
 					ContentGenerator: koji.ContentGenerator{

internal/distro/distro.go

@@ -6,6 +6,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"io/ioutil"
 	"os"
 	"sort"
 	"strings"
@@ -193,6 +194,20 @@ func GetHostDistroName() (string, bool, error) {
 	return name, beta, nil
 }
 
+// GetRedHatRelease returns the content of /etc/redhat-release
+// without the trailing new-line.
+func GetRedHatRelease() (string, error) {
+	raw, err := ioutil.ReadFile("/etc/redhat-release")
+	if err != nil {
+		return "", fmt.Errorf("cannot read /etc/redhat-release: %v", err)
+	}
+
+	//Remove the trailing new-line.
+	redHatRelease := strings.TrimSpace(string(raw))
+
+	return redHatRelease, nil
+}
+
 func readOSRelease(r io.Reader) (map[string]string, error) {
 	osrelease := make(map[string]string)
 	scanner := bufio.NewScanner(r)