Correct Dockerfile directory and install requirement first in Dockerfile

[xiangsx]

1. The Dockerfile should be placed in the project root directory for easier usage during the build process.
2. The "copy requirements.txt" step should be moved to the beginning of the Docker build process to utilize cache in case of code changes during image build.
3. The addition of the Chinese pip source has been commented out. Chinese users can uncomment it to get access to the source.