How to obtain an SSL certificate for a containerised web application - for free!

If you're reading this then it's safe to assume that I don't need to explain why it's important to secure your web service, we can just jump straight into the how. Unfortunately, a catch-all tutorial is simply not possible with the plethora of technologies within the modern developer's arsenal so I've decided upon a tech stack similar to ours at Think Engineer for this tutorial. So, a more specific title could be: How to obtain an SSL certificate, using Let's Encrypt, for a multi-container Docker web application running on Ubuntu 18.0.4 - for free!