I want to start freelancing with Rails on Upwork, but I don't have any completed production level projects in that field yet. I've been working with PHP for a few years now and have a decent portfolio, but it won't help much for Rails projects. My plan is to make a few websites using Rails in my spare time for someone for a low payment or even for free, so I can get more experience in the field and add them to my portfolio, then I would be able to find an actual job. Any advice for searching something like this? Are there platforms specifically designed for entry level coders?
You have to build it before they'll come. :)
No one is going to hire you if they can't see what you are capable of. In general, experience doesn't hold as much weight as ability. If you are a superstar in your abilities, then having little or no experience won't be that large of a hurdle. But if you can't show any abilities directly, you better have a mountain of (provable) experience before you are taken seriously.
So, like any creative field, you need to build projects. You won't get hired if you can't show what you are capable of. But I think you know that.
As for what sites to build.... I'd strongly suggest you not take on client-based projects for free, ever. If you need to build RoR sites to show off, then rework the PHP sites you've previously built. For me, that would be a big sales point --- "Here's the site build with PHP, and here's the same site with RoR." --- giving a clear indicator that you can do either. Otherwise I'd just find projects on my own, a hideous site I stumbled upon... and rebuild it purposefully showing off the skills I want to sell.
Focusing on crowd-sourcing sites in order to get work where you have no experience or projects to show is going to be a massive uphill battle. Build the portfolio, then you'll at least have some chance of gaining client work in that area.
Freelancer websites are usually for guys experienced in some field.
However, you can make you profile stating that you are junior developer giving affordable rate.
While waiting for clients, practice RoR and attach such project in portfolio. Once you get good in RoR, I am sure that clients will start hiring you.