ssh: say why when there is no shell to run

The published container image has no shell, so a session used to end at once with
nothing printed for a command. It now explains that, and what to do about it,
and exits with status 127.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

sshd/run_unix.go

@@ -25,6 +25,14 @@ func run(ch ssh.Channel, cmd string, p *ptyReq, win <-chan [2]uint32) int {
 	if shell == "" {
 		shell = "/bin/sh"
 	}
+	// Without this the client sees only a session that ends at once. The container image Bamboo is
+	// published in contains no shell at all, so say so, and what to do about it.
+	if _, err := exec.LookPath(shell); err != nil {
+		fmt.Fprintf(ch.Stderr(), "bamboo: there is no shell to run (%v).\r\n"+
+			"The Bamboo container image has none, and a shell in a container is not a shell on the machine. "+
+			"Run Bamboo directly on the machine (for example with its systemd unit) to get one, or set SHELL to a shell that exists.\r\n", err)
+		return 127
+	}
 	var c *exec.Cmd
 	if cmd == "" {
 		c = exec.Command(shell, "-l")