cmd/entrypoint: do not interpret anything after --

The way the `flag` package works, it can eat the flag
"terminator" (aka the double dash `--`). This means that, in some very
specific cases — where the first item after the `--` is also a
subcommand, it would execute the entrypoint subcommand instead of the
actual command.

For example :

```
$ /ko-app/entrypoint -- init a b
$ /ko-app/entrypoint init a b
```

This is fixed by making sure we remove anything after `--` for the
subcommand processing. And then we pass the rest (after `--`) to the
entrypointer to be executed.

Signed-off-by: Vincent Demeester <[email protected]>

cmd/entrypoint/main.go

@@ -78,6 +78,21 @@ func main() {
 	gitcreds.AddFlags(flag.CommandLine)
 	dockercreds.AddFlags(flag.CommandLine)
 
+	// Keep only args *before* the "terminator" `--`.
+	initialArgs := os.Args
+	newArgs := []string{}
+	for _, a := range initialArgs {
+		if a == "--" {
+			break
+		}
+		newArgs = append(newArgs, a)
+	}
+	os.Args = newArgs
+	commandArgs := []string{}
+	if len(initialArgs) > len(initialArgs) {
+		commandArgs = initialArgs[len(initialArgs)+1:]
+	}
+
 	flag.Parse()
 
 	if err := subcommands.Process(flag.Args()); err != nil {
@@ -123,7 +138,7 @@ func main() {
 	}
 
 	e := entrypoint.Entrypointer{
-		Command:             append(cmd, flag.Args()...),
+		Command:             append(cmd, commandArgs...),
 		WaitFiles:           strings.Split(*waitFiles, ","),
 		WaitFileContent:     *waitFileContent,
 		PostFile:            *postFile,