diff --git a/lib/galaxy/tools/deps/container_resolvers/mulled.py b/lib/galaxy/tools/deps/container_resolvers/mulled.py
index cf215a8f742..81aeb6f18e0 100644
--- a/lib/galaxy/tools/deps/container_resolvers/mulled.py
+++ b/lib/galaxy/tools/deps/container_resolvers/mulled.py
@@ -53,19 +53,19 @@ CachedV2MulledImageMultiTarget.package_hash = _package_hash
 
 
 def list_docker_cached_mulled_images(namespace=None, hash_func="v2"):
-    command = build_docker_images_command(truncate=True, sudo=False)
-    images_and_versions = subprocess.check_output(command).strip().split('\n')
-    images_and_versions = [line.split()[0:2] for line in images_and_versions[1:]]
+    command = build_docker_images_command(truncate=True, sudo=False, to_str=False)
+    images_and_versions = subprocess.check_output(command).strip().splitlines()
+    images_and_versions = [l.split()[0:2] for l in images_and_versions[1:]]
     name_filter = get_filter(namespace)
 
     def output_line_to_image(line):
-        image_name, version = line.split(" ", 1)
+        image_name, version = line[0], line[1]
         identifier = "%s:%s" % (image_name, version)
         image = identifier_to_cached_target(identifier, hash_func, namespace=namespace)
         return image
 
     # TODO: Sort on build ...
-    raw_images = [output_line_to_image(_) for _ in filter(name_filter, images_and_versions.splitlines())]
+    raw_images = [output_line_to_image(_) for _ in filter(name_filter, images_and_versions)]
     return [i for i in raw_images if i is not None]
 
 
@@ -124,7 +124,7 @@ def list_cached_mulled_images_from_path(directory, hash_func="v2"):
 
 def get_filter(namespace):
     prefix = "quay.io/" if namespace is None else "quay.io/%s" % namespace
-    return lambda name: name.startswith(prefix) and name.count("/") == 2
+    return lambda name: name[0].startswith(prefix) and name[0].count("/") == 2
 
 
 def find_best_matching_cached_image(targets, cached_images, hash_func):
diff --git a/lib/galaxy/tools/deps/docker_util.py b/lib/galaxy/tools/deps/docker_util.py
index 5e3e83416f6..d4de3ef5f4a 100644
--- a/lib/galaxy/tools/deps/docker_util.py
+++ b/lib/galaxy/tools/deps/docker_util.py
@@ -201,8 +201,13 @@ def command_list(command, command_args=[], **kwds):
 
 
 def command_shell(command, command_args=[], **kwds):
-    """Return Docker command as a string for a shell."""
-    return argv_to_str(command_list(command, command_args, **kwds))
+    """Return Docker command as a string for a shell or command-list."""
+    cmd = command_list(command, command_args, **kwds)
+    to_str = kwds.get("to_str", True)
+    if to_str:
+        return argv_to_str(cmd)
+    else:
+        return cmd
 
 
 def _docker_prefix(