Fixed sinatra error in CVE-2017-17405

[giper45]

Fix the following error:

 => ERROR [2/4] RUN gem install sinatra                                                                                                                                          1.8s
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 > [2/4] RUN gem install sinatra:                                                                                                                                                     
#0 1.227 ERROR:  Error installing sinatra:                                                                                                                                            
#0 1.227        mustermann requires Ruby version >= 2.6.0.
#0 1.228 Successfully installed ruby2_keywords-0.0.5
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failed to solve: executor failed running [/bin/sh -c gem install sinatra]: exit code: 1

Due to an update of sinatra version that is not installable in older Ruby versions.