Added the necessary files for building urlmapper into docker images

[beowulfdk]

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[chaoticm]

question: Are we going to make a new release out of it? If yes, we'd need to change the version in AssemblyInfo. IMHO this could even be 1.4.

suggestion: Add an information about what the new files are for in https://github.com/unic/SitecoreUrlMapper2/blob/main/be/docs/README.md.