ci: Use hard-coded root path for CI containers

ci/test/00_setup_env.sh

@@ -8,10 +8,18 @@ export LC_ALL=C.UTF-8
 
 set -ex
 
-# The root dir.
+# The source root dir, usually from git, usually read-only.
 # The ci system copies this folder.
-BASE_ROOT_DIR=$( cd "$( dirname "${BASH_SOURCE[0]}" )"/../../ >/dev/null 2>&1 && pwd )
-export BASE_ROOT_DIR
+BASE_READ_ONLY_DIR=$( cd "$( dirname "${BASH_SOURCE[0]}" )"/../../ >/dev/null 2>&1 && pwd )
+export BASE_READ_ONLY_DIR
+# The destination root dir inside the container.
+if [ -z "${DANGER_RUN_CI_ON_HOST}" ] ; then
+  # This folder only exists on the ci guest and will be a copy of BASE_READ_ONLY_DIR
+  export BASE_ROOT_DIR="/ci_container_base"
+else
+  # This folder is equal to BASE_READ_ONLY_DIR and is read-write
+  export BASE_ROOT_DIR="${BASE_READ_ONLY_DIR}"
+fi
 # The depends dir.
 # This folder exists only on the ci guest, and on the ci host as a volume.
 export DEPENDS_DIR=${DEPENDS_DIR:-$BASE_ROOT_DIR/depends}