fix: manually filter for dangling images for older dockers

Old versions of docker do not correctly filter out dangling images with
dangling=false, so we have to do it manually.

opensafely/pull.py

@@ -139,6 +139,9 @@ def get_local_images():
     for line in ps.stdout.splitlines():
         if not line.strip():
             continue
+        # dangling=false does not work on older versions of docker, so manually do it
+        if "<none>" in line:
+            continue
 
         line = line.replace("ghcr.io/opensafely-core/", "")
 

tests/test_pull.py

@@ -101,6 +101,16 @@ def test_default_with_local_images(run, capsys):
     ]
 
 
+def test_default_with_old_docker(run, capsys):
+    run.expect(["docker", "info"])
+    expect_local_images(run, stdout="ghcr.io/opensafely-core/r:<none>=sha")
+
+    pull.main(image="all", force=False)
+    out, err = capsys.readouterr()
+    assert err == ""
+    assert out.strip() == "No OpenSAFELY docker images found to update."
+
+
 def test_specific_image(run, capsys):
     run.expect(["docker", "info"])
     expect_local_images(run)