Add root Dockerfile so CI docker build . succeeds

[echska]

Motivation

• CI workflows run docker build . --file Dockerfile from the repository root but there was no root-level Dockerfile, causing failed to read dockerfile: open Dockerfile: no such file or directory.

Description

• Add a repository-root Dockerfile that builds the Flask backend located at parental-control-system/backend by copying requirements.txt, installing dependencies with pip, copying the backend source, exposing port 5000, and running gunicorn --bind 0.0.0.0:${PORT:-5000} main:app --workers 2 --threads 4 --timeout 60.

Testing

• Attempted a local docker build . --file Dockerfile, which could not run in this environment because the Docker CLI is unavailable (docker: command not found).

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Codex Task