Update intro text

index.md

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 site: sandpaper::sandpaper_site
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-Welcome to learn how to use Docker containers in the context of the CMS open data. This is not a full introduction to Docker, however, we do take time to explain some key concepts that should make your experience a smooth one.
+Welcome to learn how to use Docker containers as a working environment during the Midsummer QCD school. Containers will be used during the hands-on lessons on MC generators, maching learning and CMS open data. 
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+In this lesson, you will be asked to download the containers that will facilitate your work for the ML lesson and for CMS open data, and learn the basics of their use. During the school, you might need to download other containers, but once you worked through this lesson, you will find it easy.
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+This is not a full introduction to Docker, however, we do take time to explain some key concepts that should make your experience a smooth one.