Hey everyone! I'm new to both Linux and Docker and I'm hitting a few conceptual walls. I'm currently learning how to run pre-built containers, and I came across a snippet in a `docker-compose.yml` file from a third-party project. Specifically, the file defines a container named `db`, and I want to clarify the purpose of the `user` directive in this context. The relevant part of the configuration looks like this: `user: "1000:1000"`.
My main question is: does this mean that the image creator has included a root user (UID 0) and a secondary non-root user with UID 1000? I've read that it's important not to run containers as the root user, and I'm guessing that the `user: "1000:1000"` setting is advising that the container be run using the lower-privilege user instead.
But here's where it gets tricky for me: if the container writes data to a host directory like `./mysql/data`, it needs to access that folder using UID 1000, right? This raises a question: why would the author assume that the host OS has a user with UID 1000 that has access to that folder? What if this UID 1000 belongs to someone entirely unrelated to the project? I'm concerned about this if I eventually create my own container images for others.
I investigated user namespace mapping, but it seems complex for casual users, who might just want to run `docker compose up`. What am I missing in understanding this setup? Thanks for any insights!
4 Answers
Typically, UID 1000 is the first user created on a Linux system, and the image itself doesn’t make any assumptions. Thanks to user namespaces, the UID and GID in the container can be independent of the host, meaning you can have whatever setup you need. But for your volume mounts, the UID needs to align with the container to handle file permissions correctly. Any mismatch could require you to adjust host permissions yourself. It’s expected that tech-savvy users know how to manage these settings.
UID 1000 is basically a convention in Linux. The container doesn’t assume the host has a user with that ID; it just uses UID 1000 for whatever user is defined in the image. In Linux, user names are just for our visual reference, while the actual permissions are determined by UID and GID. Containers can have their own users independent of host users, so user 'abc' in the container could correlate to user 'xyz' on the host, yet they can both have a UID of 1000—this works without issues. If there’s no matching username in the container, Linux just shows the numeric ID instead. So you can run a container as 1000:1000, and it usually works fine, even without that UID existing in the container's system.
That makes a lot of sense—thanks for clarifying!
As for your concern about UID conflicts, if the root user has UID 1 and your container uses UID 1 as well, and you mount the root of the filesystem, yes, it could potentially give the container elevated access. It's important to think about how you set up permissions if you're using shared systems!
A lot of times, the first user will have UID 1000, but technically you could set it to anything. It's a general rule that doesn’t bind you, especially on systems where multiple accounts are in play. So if your users are knowledgeable enough to create accounts, they should already be slightly familiar with Docker's configuration or adjusting UID mappings.
For context, the UID/GID to username mappings come from `/etc/passwd` and `/etc/group`. There’s no connection between those on the host and inside the container. Unless you bind-mount `/etc/passwd`, they will be different. Permissions are tied to UIDs, not user names, so you can use any UID without needing it predefined in your container.