diff --git a/doc/hacking.rst b/doc/hacking.rst
index ed604a3edace..eb0ebba7114b 100644
--- a/doc/hacking.rst
+++ b/doc/hacking.rst
@@ -98,6 +98,19 @@ For integration tests, we use a system similar to `Cram tests
 
 .. _ppx_expect:      https://github.com/janestreet/ppx_expect
 
+When running dune inside tests, the ``INSIDE_DUNE`` environment variable is set.
+This has the following effects:
+
+* Change the default root detection behaviour to use the current directory
+  rather than the top most ``dune-project`` / ``dune-workspace`` file.
+* Be less verbose when Dune outputs a user message.
+* Error reporting is deterministic by default.
+* Prefer not to use a diff program for displaying diffs.
+
+This list is not exhaustive and may change in the future. In order to find the
+exact behaviour, it is recommended to search for ``INSIDE_DUNE`` in the
+codebase.
+
 Guidelines
 ----------
 
diff --git a/test/blackbox-tests/test-cases/github8041.t b/test/blackbox-tests/test-cases/github8041.t
index 1276dd4dc122..8c5a39835f43 100644
--- a/test/blackbox-tests/test-cases/github8041.t
+++ b/test/blackbox-tests/test-cases/github8041.t
@@ -1,12 +1,14 @@
   $ cat > dune-project << EOF
-  > (lang dune 1.0)
+  > (lang dune 2.4)
   > (package
   >  (name p))
   > EOF
 
   $ cat > dune << EOF
+  > (rule (copy data.txt data2.txt))
+  > 
   > (install
-  >  (files data.txt)
+  >  (files data.txt data2.txt)
   >  (section share))
   > EOF
 
@@ -15,3 +17,12 @@
 If sendfile fails, we should fallback to the portable implementation.
 
   $ strace -e inject=sendfile:error=EINVAL -o /dev/null dune build @install
+  Error: _build/default/data2.txt: Permission denied
+  -> required by _build/default/data2.txt
+  -> required by _build/install/default/share/p/data2.txt
+  -> required by _build/default/p.install
+  -> required by alias install
+  [1]
+
+#8210: data2.txt is copied from readonly-file data.txt (#3092), so it needs to
+be adequately unlinked before starting the fallback.