Allow to override build date with SOURCE_DATE_EPOCH

to make builds reproducible.
See https://reproducible-builds.org/ for why this is good
and https://reproducible-builds.org/specs/source-date-epoch/
for the definition of this variable.

This patch was done while working on reproducible builds for openSUSE.

doc/rest_docs.go

@@ -21,6 +21,7 @@ import (
 	"os"
 	"path/filepath"
 	"sort"
+	"strconv"
 	"strings"
 	"time"
 
@@ -123,7 +124,14 @@ func GenReSTCustom(cmd *cobra.Command, w io.Writer, linkHandler func(string, str
 		buf.WriteString("\n")
 	}
 	if !cmd.DisableAutoGenTag {
-		buf.WriteString("*Auto generated by spf13/cobra on " + time.Now().Format("2-Jan-2006") + "*\n")
+		now := time.Now()
+		if epoch := os.Getenv("SOURCE_DATE_EPOCH"); epoch != "" {
+			unixEpoch, err := strconv.ParseInt(epoch, 10, 64)
+			if err == nil {
+				now = time.Unix(unixEpoch, 0)
+			}
+		}
+		buf.WriteString("*Auto generated by spf13/cobra on " + now.Format("2-Jan-2006") + "*\n")
 	}
 	_, err := buf.WriteTo(w)
 	return err