Terraform and ansible. Script service configuration and use source control. Containerize services where possible to make them system agnostic.

Flatpaks are similar, but more aimed at desktop applications. Docker containers are made for services and give more isolation on the network.

Docker containers get their own IP addresses, they can discover each other internally, you get port forwarding, etc. Additionally you get volume mounts for persistent storage and other features.

Docker compose allows you to bring up multiple dependent containers as a group and manage connections between them, with persistent volumes. It’ll handle lifecycle issues (restarting crashed containers) and health checks.

An example - say you want a Nextcloud service and an immich service running on the same host. You can create two docker-compose files that launch both of them, each with its own supporting database, and give each db and application persistent volumes for storage. Your applications can be exposed to the network and the databases only internally to other containers. You don’t need to worry about port conflicts internally since each container is getting its own IP address. So those two MySQL DBs won’t conflict with each other. All you need to do is ensure that publicly available services have a unique port forwarded to them. So less to keep track of.

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Cloud backups.

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How isolated could it really be as a docker container vs a separate machine or proxmox?

You can get much better isolation with separate machines but that gets very expensive very fast.

It’s not that it provides complete isolation - but it provides enough isolation very cheaply. You still compete with other applications for compute resources but you run in your own little filesystem jail and can run that janky python version that your application needs and not worry about breaking yum. Or you can bundle that old out-of-support version of libaio that your application requires. All of your dependencies are bundled with your application so you don’t affect the rest of the system.

And since containers are standardized it allows you to move between physical computers without any modification or server setup other than installing docker or podman. You can run on Amazon Linux, RedHat, Ubuntu, etc. If it can run containers it can run your application. Containers can also be multi-platform so you can run on both ARM64 and AMD64 seamlessly.

And given that isolation you can run on a kubernetes cluster, or Amazon ECS with FARGATE instances, etc.

But that starts to get very enterprisey. For the home-gamer there is still a ton of benefit to just having file-system isolation and an easy way to run an application regardless of the local system version and installed packages. It’s a bit of an “experience” thing to truly appreciate it I suppose. Like I said - if you’ve tried running a lot of services on a system in the past without containers it gets kinda complicated rather fast. Especially if they all need databases (with containers you can spin up one db for each application easily).

But what do you actually gain from this?

Isolation. The number one reason to use docker is isolation. If you’ve not tried to run half a dozen services on a single server then this may not mean much to you but it’s a “pretty big deal.”

I have no idea how the synology app store works from this pov - maybe it’s docker under the covers. But in general I despise the idea of a NAS being anything than a storage server. So running Nextcloud, Immich, etc. on a NAS is pretty anathema to me either way.