Fix npm global install permissions in memax-dev Dockerfile

[dizys]

Motivation

• Building the image failed with EACCES when running npm install -g @openai/codex because the Dockerfile was running global npm installs as USER coder while npm's default global location is root-owned /usr/lib/node_modules.
• Several chown lines used relative paths (missing leading /) which could cause ownership changes to target incorrect locations.

Description

• Add ENV NPM_CONFIG_PREFIX="/home/coder/.local" so global npm installs write into a user-writable location when running as coder.
• Fix chown paths for .codex, .gemini, and .copilot to use absolute /home/coder/... paths.
• Preserve existing creation and ownership of /home/coder/.local/bin so the new npm prefix is usable by the coder user.

Testing

• Ran git diff --check to validate there are no whitespace or patch issues and it passed.
• Inspected the updated Dockerfile segment with nl -ba to confirm the env and chown changes are present and correct.
• Did not run a full docker build in this environment; the change is a configuration fix that addresses the observed EACCES install failure.

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Codex Task