fix logic and tests, update docker CI file

app/Mail/TeacherSubmissionReminderMail.php

@@ -0,0 +1,25 @@
+<?php
+
+namespace App\Mail;
+
+use Illuminate\Bus\Queueable;
+use Illuminate\Mail\Mailable;
+use Illuminate\Queue\SerializesModels;
+
+class TeacherSubmissionReminderMail extends Mailable
+{
+    use Queueable;
+    use SerializesModels;
+
+    public function __construct(
+        protected string $subjectLine,
+        protected string $htmlBody
+    ) {
+    }
+
+    public function build(): self
+    {
+        return $this->subject($this->subjectLine)
+            ->html($this->htmlBody);
+    }
+}