The docker build fails

[fabianegli]

The building of a docker image with the provided Dockerfile fails.

The reason for this is that various software lines do not contain version informations and as the default moves on to newer versions, the build fails.

The biggest change is the new official Ubuntu release 20. When diffrac was created, 18 was the official version.

An additional breakage is caused by the changes in the statsmodels package after the diffrac release, so the version of statsmodels also needs to be defined - otherwise the diffrac tools need to be adapted to the changes in statsmodels.