CI: Add build instructions for docker

Signed-off-by: Alexander Diewald <diewald@fortiss.org>

Readme.md

@@ -7,6 +7,16 @@
   submodule (see .gitmodules) must contain a generic pom.xml file to enable
   pomless builds. Please see https://git.fortiss.org/af3/af3.git for an example
   of such a pom file.
+* docker
+  Contains the build specification for the docker container used in the CI Jobs.
+  Build & Update instructions:
+  * Clone the repository.
+  * Navigate to the cloned directory in a UNIX shell and apply the following
+    commands.
+  * `docker login git.fortiss.org:5001 -u <USERNAME>`
+  * `docker build -t git.fortiss.org:5001/af3/maven-releng docker/`
+  * Delete the outdated 'af3/maven-releng' container
+  * `docker push -t git.fortiss.org:5001/af3/maven-releng`
 * features
   Holds the feature descriptions that compose a product. Each source bundle must
   be listed here.