title: Jinja2 - Expressions concatenating issue
body: <p>I've got this variable 
  <code>{% set img = Bicycles[Keys[i]][2] %}</code> which is a string
  I'm trying to add it to an img html element like this</p>
  
  <p><code>&lt;img src="{{ url_for('static', filename=' {{ img }} ') }}" class="card-img-top" alt="..."&gt;</code></p>
  
  <p>My expected output when the page loads is</p>
  
  <p><code>&lt;img src="{{ url_for('static', filename='theurlhere') }}" class="card-img-top" alt="..."&gt;</code></p>
  
  <p>But all i get is this <code>"/static/%20%7B%7B%20img%20%7D%7D%20"</code></p>
  
  <p>I'm so lost e_e</p>