Docker deployment gets "Mixed content" error on installation page

I’m trying to deploy OJS to Render.com using Docker runtime. The installation page doesn’t load properly and it has lots of “Mixed Content” errors.

I’m using this one-line Dockerfile

FROM pkpofficial/ojs:lts-3_3

I’m supplying the SERVERNAME env variable with render-ojs-lts.onrender.com.

There’s no error in build & deployment log. Here’s the site https://render-ojs-lts.onrender.com/

Please see the screenshot below:

What did I miss? I am not aware there’s a deployment guide with docker. I’m happy to read any articles / forum thread.

Thanks in advance.

Additional info
I’m new to OJS codebase and PHP. However, I had years of experience as web developer. I found similar issues in this forum but it wasn’t clear if they’re using docker or not. Therefore, I’m writing a new post here.