Not sure if you are the one to report this to but I tried using your template and assumed the docker uses postgres by default for a db... in the log starting firefly i noticed this errror:
Run various artisan commands...
.deploy/docker/entrypoint.sh: line 53: /mnt/cache/appdata/firefly-iii//.deploy/docker/wait-for-it.sh: No such file or directory

I dont think the default value is correct or mine is not correct "/mnt/cache/appdata/firefly-iii/" as that is where i keep my appdata for my dockers but it appears that is creating that location in the docker container which doesnt seem correct.

The template is using 5080:5080 even though the description for the container port is 80 which is still incorrect. I should be 8080:5080. Took me a while to realize this, wondering why the gui wasnt coming up.