images/universal: preserve upstream user environment when switching to coder

[matifali]

Summary

Rename the upstream codespace user to coder instead of deleting it and creating a fresh account.

Why

The universal image is built on top of mcr.microsoft.com/devcontainers/universal, which is configured around the existing codespace user and its home directory. Recreating the user leaves behind shell and PATH behavior that still assumes /home/codespace.

In Coder workspaces this can surface as login shells dropping the injected coder CLI from PATH, causing coder: command not found in some terminal flows.

I reproduced that behavior in a real workspace from the Docker template:

• bash -c 'command -v coder' works
• bash -lc 'command -v coder' fails

Preserving the upstream account avoids that mismatch and keeps the base image's home directory and shell environment coherent.

Changes

• replace userdel -r codespace + useradd coder with usermod/groupmod
• move the home directory to /home/coder
• keep coder in the docker and sudo groups
• update the universal image README to describe the approach

Expected result

Login-shell behavior remains consistent with the upstream devcontainer image while still exposing the user as coder for Coder workspaces.

[matifali]

Closing in favor of #332, which uses a direct branch in coder/images instead of the fork branch.

The direct PR also includes the local Docker CLI validation and a follow-up fix for login shells preserving Coder's injected PATH entries.

🤖 This response was generated by Coder Agents.