Dockerfile best practice: specify non-root USER in custom tool example Dockerfile

[neubig]

Context

The Dockerfile at examples/02_remote_agent_server/06_custom_tool/Dockerfile does not currently specify a USER directive, so the container runs as root by default.

Best practice

It is a Docker best practice to run containers as a non-root user. Adding a USER directive with a non-root user avoids unnecessary privilege and follows community conventions for well-formed Dockerfiles.

Suggested change

Add a non-root user to the Dockerfile, e.g.:

RUN useradd -m appuser
USER appuser

Or use an existing non-root user if one is already created in the base image.

Acceptance criteria

• Dockerfile includes a USER directive with a non-root user
• Container still builds and runs correctly

[github-actions]

⚠️ Not yet ready-for-dev

This issue does not yet meet the readiness criteria, so the
ready-for-dev label has not been applied (or has been removed).
The criteria are type-specific:

Bug reports (bug label, see .github/ISSUE_TEMPLATE/bug_template.yml):

• The ### Actual Behavior section must describe reproducible SDK steps
  and include a command such as python, pytest, uv, or pip.
• An ### Acceptance Criteria section with at least one checklist item (- [ ] …).

Enhancements (enhancement label, see .github/ISSUE_TEMPLATE/feature_request.yml):

• A ### Desired Behavior section.
• An ### Acceptance Criteria section with at least one checklist item.

What is missing:

• The issue has neither the bug nor enhancement label, so its readiness criteria cannot be evaluated. Add the appropriate label.

Edit the issue to address the gaps and the ready-for-dev label will be
applied automatically.

Note: Section headings require exact ### matches from the templates.
If a missing type label is reported, no action is needed from external
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