Univention Corporate Server 4.1 erratum 27 (bugfix)

This update addresses the following issues:
* The check whether docker is up and running at the startup of the
  App Center can now be disabled by setting the UCR variable
  appcenter/docker=disabled. In this case, Docker Apps cannot be
  installed anymore.
* Added a new command for univention-app: univention-app shell runs a command
  within the environment of a Docker App.
* Warnings shown before App installations do not
  show up as errors after a successful installation anymore.
* Failed Docker App installations now correctly remove their Docker
  containers when cleaning up.
* Downloading specific files from the App Center server now makes use of
  ETags, resulting in a speedup of module opening.