User-Management.md

+## Management account: `gitrunner`
+- User which runs the `gitlab-runner`
+- Should have a local (not in the filesystem) directory for account mappings and logs (e.g. `/home/gitrunner/local`)
+
+## CI group:
+- A Linux group `ci` containing the `gitrunner` account as well as each CI service account
+- Should have a global (in the filesystem) directory which includes (e.g. `/home/ci/`)
+  - `gitlab-runner`-executable (must be accessible from each CI service account)
+  - Helper scripts of the `CustomDriver` repo `core/scripts`
+
+## CI users:
+- Should have (r+x)-access to `/home/ci/.local/bin`
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