Add proper informations about Docker build. May close #119 (PR #120)

Hi,

As there are some break changes at the last major update of GCC and clang version 10, I suggest documenting a little bit more Docker building, at least as a temporary solution. (See https://github.com/GrangerHub/tremulous/issues/119)
As a great work were already done, it works out the box seamlessly. I just add some explanations about usage.

GitHub

The title of this pull request changed from “Add proper informations about Docker build. May close https://github.com/GrangerHub/tremulous/issues/119” to "Add proper informations about Docker build. May close #119

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