Modify Dockerfile, Rearrange Configurations and Remove redundancy in start.sh

[Karl-Han]

For the latter 2 commits:

Changes are the followings:

1. Removing all the lines in start.sh to overwrite the existing /bot, /root, etc.
2. Do all the configurations in the Dockerfile instead of start.sh and start.sh should only do the initialization for the container.

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For the first 6 commits:

Changes are the followings

1. Add .gitignore with the Python version to avoid unwanted files
2. Rearrange configurations to make it maintainable
3. Modify Dockerfile to
   • reduce the number of retrieving packages from Ubuntu Mirror and PyPI
   • comply with the modification of configurations