fix(docker): redeclare build args inside runtime stage for buildx

Redeclare TTYD_VERSION and DOCKER_CLI_VERSION after FROM node:22 so both variables are guaranteed to be available in stage RUN instructions with buildx/docker-container driver.

This prevents Docker CLI download URLs from resolving with empty or missing version values in CI and avoids false 404 failures during image build.

refactor: switch runtime mode selection to RUN_MODE env var

Replace positional startup mode arguments with RUN_MODE-based behavior and default to CLI mode. Update compose configuration and smoke test expectations accordingly. Refresh README with updated usage and setup examples.

fix: ttyd command quotes removed, workdir added

refactor: Improve user handling and switch to Debian base image

This commit introduces several key improvements to the Docker image configuration and entrypoint script to enhance stability and robustness.

The base image has been switched from Alpine to the official Debian-based Node.js image (`node:22`). This resolves intermittent crashes observed with `gemini-cli` during shell
command execution on Alpine Linux, providing a more stable runtime environment.

The entrypoint script has been significantly refactored to handle user and group ID management more intelligently. Instead of failing or taking over existing UIDs/GIDs, the script
now remaps any conflicting user or group to a new, unused ID. This prevents permissions errors and makes the container more adaptable to different host environments.

Additionally, the unused `WORKSPACE` environment variable has been removed from the Dockerfile and documentation to simplify the configuration.

Fix parameter handling with quotes in gosu command

Wrap the command in a bash -c string using careful quoting and positional parameters.
This ensures that arguments containing quotes are correctly passed to the GEMINI_CMD.

Ensure GEMINI_HOME directory exists before user creation

Added logic to create the GEMINI_HOME directory if it does not exist before creating the Gemini user.