Changed docs Dockerfile to start with yarn instead of npm

[danias]

Proposed changes

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Types of changes

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• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

Checklist

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• I have read the CONTRIBUTING doc
• I have signed the Bitloops Language CLA (if not, email us: [email protected])
• I created tests which fail with the change
• Code complies correctly
• Unit tests pass locally with my changes
• All existing tests passed
• I have added tests that prove my fix is effective or that my feature works
• I have added/extended necessary documentation
• Any dependent changes have been merged and published in downstream modules

Further comments

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