testing: ensure run.sh is executing in docker container

On the host, some undesired things could end up happening if this
is executed, so we now exit and log if that is attempted.  In addition,
we now backup the previous cargo config if present.  This will never
be the case in the current build setup, but that could change if
we decide to do builds in a data volume in the future.

Signed-off-by: Paul Osborne <osbpau@gmail.com>

ci/run.sh

@@ -5,6 +5,16 @@
 
 set -e
 
+# This should only be run in a docker container, so verify that
+if [ ! -f /.dockerinit ]; then
+    echo "run.sh should only be executed in a docker container"
+    echo "  and that does not appear to be the case.  Maybe you meant"
+    echo "  to execute the tests via run-all.sh or run-docker.sh."
+    echo ""
+    echo "For more instructions, please refer to ci/README.md"
+    exit 1
+fi
+
 BASE_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
 MANIFEST_PATH="${BASE_DIR}/Cargo.toml"
 BUILD_DIR="."
@@ -18,9 +28,8 @@ export RUST_TEST_THREADS=1
 # Tell cargo what linker to use and whatever else is required
 #
 configure_cargo() {
-  rm -rf .cargo
-  mkdir .cargo
-  cp "${BASE_DIR}/ci/cargo-config" .cargo/config
+  mkdir -p .cargo
+  cp -b "${BASE_DIR}/ci/cargo-config" .cargo/config
 }
 
 #