Heimdall Container - Permission Denied

With the latest container version, I had a cache issue which I found a workaround for (here).

However, now I am getting a “Permission denied” error from the page itself; it looks like the container is not able to write to /var/www/localhost/heimdall and subdirectories. I did a bit of troubleshooting, and here’s what I’m seeing:
-The owner of the container files is root
-However, the PHP user is abc, with group abc. Neither of those are part of the ownership structure for the container.
-If I run chown on a directory, it works, but that does not persist with restarts and, of course, if I clear the data to try again.

I am not comfortable enough with building a docker image to figure out how to fix this myself, but it seems like this ownership issue has something to do with it.