outposts: send build hash as part of hello

Signed-off-by: Jens Langhammer <jens.langhammer@beryju.org>

outpost/pkg/version.go

@@ -1,3 +1,16 @@
 package pkg
 
+import (
+	"fmt"
+	"os"
+)
+
 const VERSION = "2021.5.1-rc7"
+
+func BUILD() string {
+	return os.Getenv("GIT_BUILD_HASH")
+}
+
+func UserAgent() string {
+	return fmt.Sprintf("authentik-outpost@%s (%s)", VERSION, BUILD())
+}