Make efficient use of Maven caching in CI and Docker builds

[poikilotherm]

This is related to my other dependency housekeeping things. See #5288 and #5360.
Also related to gdcc/dataverse-kubernetes#64.

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1. Maven downloads the world for dependencies and again for plugins. These downloads should be avoided when the POM is unchanged.
   • Clarification: Maven will cache this locally, e. g. in a .m2/repo dir, but in ephemeral build envs, this will be lost after each build.
2. All modern CI tools like Travis, Jenkins, GitLab, ... offer caching for reuse during next build.
3. Docker layers offer caching.

To efficiently cache all dependencies and plugins, one needs to tell Maven it shall "download the world" before executing anything else. After that, Maven can be used in offline mode: mvn -o ....

However, the official dependency plugin target mvn dependency:go-offline does NOT download everything which is necessary: plugins are missing. There are some bugs for this, see MDEP-82, MDEP-516 and many more, but this has not been resolved yet.

For further enhancement of our CI and Docker efforts, I would like to add a Maven plugin. It will do no harm to anyone else, as you need to explicitly use it. Otherwise it will do just nothing.
https://github.com/qaware/go-offline-maven-plugin