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If you want to test it locally use dpkg -i command to install a package. Also you can download some dependency packages required by your new package from du2 repository using apt download command in a zone with active du2 repository, then just install them into the test zone. For example, you could install some gcc-6 libraries, in some time they will be replaced by gcc-8.
You may want to create local repository, see HOWTO page https://dilos-dev.atlassian.net/wiki/spaces/DS/pages/2323775489/How+to+create+local+repository+in+the+zone