Selfhosted

First of all let me make this absolutely clear, docker is not expected to be secure to that level. While they try to make it hard for someone to escape a container, it's not their main concern so expect that there are vulnerabilities that would allow an attacker to escape.

Now the second thing, the Overseer login screen might be secure enough for your case, the problem is that login is hard to do right, and Overseer are doing several other stuff as well, so they might not give it enough emphasis, and even if they do, maybe Immich devs don't, or any one of the dozens of other services, so there are dozen of possible points of failure. Things like Authelia or Google OAuth are focused on authentication, so they do that absolutely right, and then they become the only point of failure for authentication.

To be fair, if you keep things updated it's unlikely not having auth would be a problem. Mostly because most hackers won't even know of your server to begin with. And most systems are secure enough for most casual hacks. But it's an investment worth the time if you plan on making something available to the internet.