Wait for docker containers to exist before starting recording.

This enables us to capture the full life of the container since the script can now be kicked off before the container is launched.

docker/scripts/graph_docker_memory.py

@@ -30,6 +30,12 @@ def main():
     logging.basicConfig(level=logging.INFO)
     samples: list[Sample] = []
     labels: dict[ContainerId, ContainerName] = {}
+    # First wait for any docker container to exist.
+    while True:
+        sample, labels_in_sample = take_sample()
+        if labels_in_sample:
+            break
+    # And then record memory until no containers exist.
     while True:
         sample, labels_in_sample = take_sample()
         if not labels_in_sample: