ci: Extend docs

ci/README.md

@@ -8,6 +8,16 @@ and numbered according to which stage and lifecycle step it belongs to.
 
 ### Running a stage locally
 
+Be aware that the tests will be built and run in-place, so please run at your own risk.
+If the repository is not a fresh git clone, you might have to clean files from previous builds or test runs first.
+
+The ci needs to perform various sysadmin tasks such as installing packages or writing to the user's home directory.
+While most of the actions are done inside a docker container, this is not possible for all. Thus, cache directories,
+such as the depends cache or ccache, are mounted as read-write into the docker container. While it should be fine to run
+the ci system locally on you development box, the ci scripts can generally be assumed to have received less review and
+testing compared to other parts of the codebase. If you want to keep the work tree clean, you might want to run the ci
+system in a virtual machine with a Linux operating system of your choice.
+
 To allow for a wide range of tested environments, but also ensure reproducibility to some extent, the test stage
 requires `docker` to be installed. To install all requirements on Ubuntu, run
 
@@ -26,6 +36,3 @@ To run the test stage with a specific configuration,
 ```
 FILE_ENV="./ci/test/00_setup_env_arm.sh" ./ci/test_run_all.sh
 ```
-
-Be aware that the tests will be build and run in-place, so please run at your own risk.
-If the repository is not a fresh git clone, you might have to clean files from previous builds or test runs first.