remove space in arg assignment (docker example)

[axkirillov]

Changes

• adjust the "add blender to docker" example

Running this example prior to this pr would return a parse error "ARG names can not be blank"

Checklist

• [x] Read the contribution guide and accept the code of conduct
• [x] Readme (updated or not needed)
• [x] Tests (added, updated or not needed)

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