Updating docker file and docs for using github PAT when downloading Unreal Engine

[DanielAtt2000]

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Description

GitHub doesn't support passwords anymore and therefore a Personal access token needs to be used. The appropriate changes where made both in the docs and in the dockerfile.

Fixes #

Where has this been tested?

• **Platform(s):Linux 18.04 LTS ...
• **Python version(s):3.6.9 ...
• **Unreal Engine version(s):4.26 ...

Possible Drawbacks

None

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This change is 

[update-docs[bot]]

Thanks for opening this pull request! The maintainers of this repository would appreciate it if you would update our CHANGELOG.md based on your changes.

[Bonifatius94]

I wouldn't name it GIT_USER. Instead I would use GITHUB_USER_WITH_UE_ACCESS to make clear that it's about a GitHub account that's allowed to download the Unreal Engine's source code.

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.