Allow DOCKER_HOST override in rootless Docker setup

[f3l1x]

Summary

This change makes the rootless Docker configuration respect an existing DOCKER_HOST environment variable, allowing users to override the default socket location or use alternative Docker daemons (mounted host sockets, remote daemons, etc.).

Key Changes

• dockerd-rootless-start.sh:

  • Modified to respect pre-existing DOCKER_HOST environment variable instead of always overwriting it
  • Added early check to detect if a daemon is already reachable at the configured DOCKER_HOST
  • Added validation to ensure DOCKER_HOST points to a unix socket path ending in /docker.sock
  • Derives XDG_RUNTIME_DIR from the DOCKER_HOST path to keep the daemon and CLI in agreement
  • Provides helpful error messages when DOCKER_HOST is set to an unsupported value

• Dockerfile:

  • Updated /etc/profile to use DOCKER_HOST="${DOCKER_HOST:-...}" pattern, making the default conditional
  • Added clarifying comments explaining that these are defaults that can be overridden

• README.md:

  • Documented that both XDG_RUNTIME_DIR and DOCKER_HOST are defaults only
  • Explained that environment-provided values (from templates, docker run, mounted sockets, remote daemons) are respected
  • Clarified the behavior of dockerd-rootless-start when DOCKER_HOST is pre-configured

Implementation Details

The startup script now follows this logic:

1. Use DOCKER_HOST from environment if set, otherwise default to the rootless socket path
2. Check if a daemon is already reachable at that location
3. If reachable, exit successfully without starting a new daemon
4. If not reachable, validate that DOCKER_HOST is a unix socket path and derive XDG_RUNTIME_DIR from it
5. Create the runtime directory if needed and start the rootless daemon

This enables flexible deployment scenarios while maintaining backward compatibility with existing setups.

https://claude.ai/code/session_017jPxeenJt19PJStMMeb1Em