Update docker

pages/Docker.md

@@ -7,3 +7,17 @@ Topics:
   - archiving of software
 status: unedited
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+
+Due to the huge variety of operating systems, languages and frameworks, the
+number of possible system configurations has grown rapidly in the past decades.
+Operating-system-level virtualisation is one of the most successful techniques
+to tackle this problem and allows the conservation of environments, making them
+interoperable and reproducible in an almost system agnostic way. KM3NeT utilises
+Docker (https://www.docker.com) for this task, which is the most popular
+containerisation solution with high interoperability. Docker containers run with
+negligible performance overhead and create an isolated environment in a fully
+reproducible manner, regardless of the host system as long as Docker itself is
+supported (Linux, macOS and Windows).
+These containers are used in the GitLab CI to run test suites in many different
+configurations. Python based projects for example can easily be tested under
+different Python versions.