Add CI instructions in docs/

docs/for_devs/ci.rst

+CI Pipeline
+==================================
+
+Building the Docker Image
+-------------------------
+Sassena uses a docker container to build the program and run the unit tests on
+the CI server.
+
+The docker image can be build on a local machine using the command::
+  docker build -t registry.hzdr.de/daphne4nfdi/sassena .
+
+If you have made changes to the docker container and want to push it to the CI,
+simply update the Sassena registry using the command::
+  docker push registry.hzdr.de/daphne4nfdi/sassena