Is a portfolio website necessary for a junior backend developer and how can one showcase their skills as a backender?

I've been a professional developer for 26 years, working for many different companies, freelancing, hiring my own teams, etc. and... to be perfectly honest, a portfolio website is not really necessary for a developer. An interesting and active GitHub (or similar) account is a far better way to judge someone's skills/competency than a curated vanity project. The honest 'warts and all' approach is way better

Portfolio websites are optional now because we can just use a GitHub readme page for a portfolio. An active GitHub with lots of projects and code should more than suffice.

Showing backend skills is super easy. Just create a backend API using some sort of tech stack like JavaScript and Python. Hook it up to a database and put it inside of a Docker container and boom! You are showcasing your ability to develop backend architectures.

Yes and no.
Yes, because you're creating your online presence and can link some communities or professional profiles (even github) to show your work.
No, because if you main development area is backend, show rainbows or dancing cats don't add anything new to your resume or skills.

As backender you could link some github repositories where your worked or tried to connect some API with a own wrapper library (ie: Wikipedia API o Pokemon API?) to show how you work with them and answer questions in StackOverflow sometimes is a plus...