cmd, internal/build, docker: advertise commit date in unstable build versions (#19522)

* add-date-to unstable

* fields-insteadof-split

* internal/build: support building with missing git

* docker: add git history back to support commit date in version

* internal/build: use PR commits hashes for PR builds

cmd/geth/main.go

@@ -50,8 +50,9 @@ const (
 var (
 	// Git SHA1 commit hash of the release (set via linker flags)
 	gitCommit = ""
+	gitDate   = ""
 	// The app that holds all commands and flags.
-	app = utils.NewApp(gitCommit, "the go-ethereum command line interface")
+	app = utils.NewApp(gitCommit, gitDate, "the go-ethereum command line interface")
 	// flags that configure the node
 	nodeFlags = []cli.Flag{
 		utils.IdentityFlag,