bench: add ephemeral full-lifecycle scenario

bench/scenarios/22-ephemeral-true.sh

@@ -0,0 +1,11 @@
+SCENARIO_DESCRIPTION="distrobox ephemeral alpine -- /bin/true (full lifecycle)"
+SCENARIO_WARMUP=1
+SCENARIO_RUNS=5
+
+EPHEMERAL_IMAGE="docker.io/library/alpine:latest"
+
+scenario_setup()   { :; }
+scenario_command() {
+    printf '%s ephemeral --image %s --yes -- /bin/true\n' "$EXEC" "$EPHEMERAL_IMAGE"
+}
+scenario_cleanup() { :; }