Allow backup to be run as non-root user (#366)

* Allow backup to be run as non-root user

* Document usage as non-root user

* Also test /etc access

* Choose better name for doc

test/nonroot/run.sh

@@ -0,0 +1,27 @@
+#!/bin/sh
+
+set -e
+
+cd "$(dirname "$0")"
+. ../util.sh
+current_test=$(basename $(pwd))
+
+export LOCAL_DIR=$(mktemp -d)
+
+docker compose up -d --quiet-pull
+sleep 5
+
+docker compose logs backup
+
+# conf.d is used to confirm /etc files are also accessible for non-root users
+docker compose exec backup /bin/sh -c 'set -a; source /etc/dockervolumebackup/conf.d/01conf.env; set +a && backup'
+
+sleep 5
+
+expect_running_containers "3"
+
+if [ ! -f "$LOCAL_DIR/backup/test.tar.gz" ]; then
+  fail "Could not find archive."
+fi
+pass "Archive was created."
+