The main appeal of PHP for me has always been it's ability to work as a “serverless” execution environment, long before this marketing concept even existed, so hosting your own PHP on a cloud machine with Docker sounds really backward to me.
I can give them resource limits the same way (CPU/memory limits, except easier than cgroups), as well as set restart policies and have a clear look at what's executing where, with something like Docker Swarm it becomes like systemd across multiple nodes and scaling up/down becomes easy, especially with load balancing for network calls. Software like Portainer also has pretty nice discoverability.
Speaking of networking, I don't have to worry about tunnels or firewall configuration myself, can just expose a web server that acts as a reverse proxy and give everything else custom private networks that span across nodes (with something like Docker Swarm again, though Consul and Kubernetes have the same functionality, details aside).
I can have custom port mappings (regardless of what the software uses, I might not even care about digging in some configuration file to change it), which is especially useful when running multiple separate instances on the same machine (like different versions of PostgreSQL, or separate instances for different projects), or hostnames in case I don't want to expose ports.
I can easily have custom persistent/transient storage paths or even in memory storage (tmpfs), when I have persistent storage then suddenly backups become easy to do and I can be very clear about all other directories being wiped and being in a known state upon startup/restart. It's also immensely useful for me to escape the sometimes weird ways how software on *nix uses the file system, I can just mount my persistent files in /app/my-app/database/var/lib/postgresql/data or /app/my-app/web-server/etc/apache2/sites-enabled and know that I don't care about anything outside of /app.
I can also treat Docker as lightweight VMs, except a bit more stateless, in that I can have container images that I base on a version of Debian/Ubuntu/Alpine or whatever, ship them, and then don't have to worry about a host OS update breaking something, because only Docker or another runtime like Podman is the actual dependency and most of the other software on the node doesn't come in contact with what I'm running. With rootless containers, that also improves the separation and security there a little bit.
With all of that in place, suddenly I can even move apps and all of their data across nodes as necessary, load balance software across multiple nodes, be able to easily tell people how to run what I have locally and store and later use these images very easily. Are there pieces of software or alternatives (e.g. jails) that do a lot of the same? Sure, but Docker essentially won in ease of use.
That's the biggest problem I see with Docker: nobody has an incentive to make well structured software with a lean dependency chain and a straightforward installation process… These used to be good proxy of the overall software quality of the project, but now Rube Goldberg projects that just happen to work by luck are routinely distributed and the user has no idea of how big of a mess it is internally.
If I tried to run all of these directly on the hardware with whatever minimal non-Docker setup each uses, I'd have a dozen update processes, a dozen different ways to start the server, and a dozen log files following a dozen different conventions for storage. I'd also have to be sure that each app I add either uses a different database and language runtime than the ones I've installed already or is compatible with the versions of those that I already installed.
Instead, with Docker/Podman, I can use the same tool (compose files stored in a git repo) to manage all of the apps and their dependencies with zero risk of weird dependency issues across app boundaries.
Same reason you'd use Docker for anything, why would it matter if it's Python, PHP or Rust?
Is there something specific about the language that makes Python (or other language) more suitable with Docker for you, compared to PHP?
(Personally I only use Docker when I start to deal with multiple hosts/distributed architecture, which doesn't happen a lot tbh)
Python has notoriously awful dependency management. One of the biggest appeals of Docker is that it lets you build the equivalent of a "fat jar" so that you get at least somewhat reproducible versions of your dependencies at runtime. For a language with decent dependency management the value proposition is much weaker.
Not that I wouldn't just use Docker for this, but those two help a lot when dealing with multiple clients who have different versions requirements.
I would ask the same question if you were using docker for a Rust project btw.
IMHO Docker mostly make sense when you have projects that require globally installed dependencies (like C or Python).
I can easily see which directories or files to back up, and it's fairly explicit which knobs I've tweaked or config files I've changed, regardless of what stack the app relies on.
It's also makes it much easier to roll back a version. Just take zfs snapshots of relevant directories before pulling new image, if it goes south just roll back snapshots and use the old image.
What about keeping the machine up to date to new distro release that inevitably comes with a new version of PHP that isn't compatible with the app ?
Don't get me started on setting properly php-fpm and any other reverse proxy.
All of those issue are gone with docker. You always run the right version of everything as it was intended by the developer (if they are the on that maintain the image)
Docker has its advantages, but the approach also has a lot of disadvantages which are not so obvious to junior developers.
Isolation seems fun, but the interfaces (Unix sockets where anything goes) are extremely brittle. Version management seems simple at first, but will become horrible once old containers offer no upgrade path in the future, or when the free hubs from today will become tomorrow's subscription model.
I'm not advocating for PHP, but it sure made deployment of several websites on one machine extremely simple. Eventually version management destroyed some of the fun, which will probably happen with Docker containers as well, given enough time.
Java's application servers were initially also hailed with similar enthusiasm as Docker containers, and look at the complicated mess that has become.
To some, all that is old is new again.
