chore: run dev containers as non-root users

[vee1e]

Run dev containers as non-root users

Description

• Added non-root runtime users in backend and webapp dev Dockerfiles.
• Set writable ownership for backend app/output paths, then switched runtime to USER non-root.
• Kept startup commands unchanged to preserve dev workflow.

Additional context

All test cases pass

docker build -f gdbui_server/DockerFile.dev -t gdbui-server:issue-71 gdbui_server  # pass
docker build -f webapp/DockerFile.dev -t gdbui-webapp:issue-71 webapp              # pass
docker run --rm gdbui-server:issue-71 id -u                                        # 999
docker run --rm gdbui-webapp:issue-71 id -u                                        # 1000
docker run --rm gdbui-server:issue-71 sh -lc 'python -c "from pathlib import Path; Path(\"output/_perm_test.tmp\").write_text(\"ok\"); print(\"ok\")"'  # ok