[Bug]: Switching user during Archlinux's pacman RUN statement fails

### I have done the following

- [x] I have searched the existing issues
- [ ] If possible, I've reproduced the issue using the 'main' branch of this project

### Steps to reproduce

1. On an existing installation from brew
2. Create an Containerfile with the following contents:
```Containerfile
FROM docker.io/library/archlinux:latest

RUN pacman -Sy --noconfirm fzf

```
3. Run `container build --arch amd64 .`
4. Observe the reproducible error from the TTY log

### Problem description

During package installation, Archlinux's pacman package manager switched to the `alpm` user. This leads to the following error:

```
[+] Building 0.2s (5/5) FINISHED
=> [resolver] fetching image...docker.io/library/archlinux:latest 0.0s
=> [internal] load build definition from Dockerfile 0.0s
=> => transferring dockerfile: 2B 0.0s
=> [internal] load .dockerignore 0.0s
=> => transferring context: 2B 0.0s
=> CACHED oci-layout://docker.io/library/archlinux:latest@sha256:a9a1e6db522cf5f8018c920d035e2 0.0s
=> => resolve docker.io/library/archlinux:latest@sha256:a9a1e6db522cf5f8018c920d035e2bfbf8e39c 0.0s
=> ERROR [linux/amd64 1/2] RUN pacman -Sy --noconfirm fzf 0.2s
------
> [linux/amd64 1/2] RUN pacman -Sy --noconfirm fzf:
0.146 :: Synchronizing package databases...
0.157 error: error restricting syscalls via seccomp: 22!
0.157 error: switching to sandbox user 'alpm' failed!
0.158 error: failed to synchronize all databases (failed to retrieve some files)
------
Error: unknown: "failed to solve: process "/bin/sh -c pacman -Sy --noconfirm fzf" did not complete successfully: exit code: 1"
```

### Environment

```markdown
- OS: macOS 26.5 (25F71)
- Xcode: Xcode 26.5 (17F42)
- Container: container CLI version 0.12.3
```

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

Reproduce the failure with the reported Arch Linux Containerfile and `container build --arch amd64 .` on the existing macOS installation. Trace the build path for the RUN statement, focusing on seccomp setup and switching to the `alpm` user. Done means the pacman command completes successfully without the syscall-restriction or sandbox-user errors.

Written by the indexing model from the issue text.