restructure to make roundtrip testable

The combination

- preprocess_volumes which creates Volume objects and creates strings
  again in the end and
- creation of DockerVolumes which are only temporary and are soon
  converted to str again

confused me and needed testing IMO. Also code was redundant.

lib/galaxy/tool_util/deps/container_classes.py

@@ -406,6 +406,18 @@ def add_var(name, value):
         return volumes_str
 
 
+def _parse_volumes(volumes_raw: str, container_type: str) -> List[DockerVolume]:
+    """
+    >>> volumes_raw = "$galaxy_root:ro,$tool_directory:ro,$job_directory:ro,$working_directory:z,$default_file_path:z"
+    >>> volumes = _parse_volumes(volumes_raw, "docker")
+    >>> [str(v) for v in volumes]
+    ['"$galaxy_root:$galaxy_root:ro"', '"$tool_directory:$tool_directory:ro"', '"$job_directory:$job_directory:ro"', '"$working_directory:$working_directory:z"', '"$default_file_path:$default_file_path:z"']
+    """
+    preprocessed_volumes_list = preprocess_volumes(volumes_raw, container_type)
+    # TODO: Remove redundant volumes...
+    return [DockerVolume.from_str(v) for v in preprocessed_volumes_list]
+
+
 class DockerContainer(Container, HasDockerLikeVolumes):
     container_type = DOCKER_CONTAINER_TYPE
 
@@ -445,9 +457,7 @@ def containerize_command(self, command: str) -> str:
             raise Exception(f"Cannot containerize command [{working_directory}] without defined working directory.")
 
         volumes_raw = self._expand_volume_str(self.destination_info.get("docker_volumes", "$defaults"))
-        preprocessed_volumes_list = preprocess_volumes(volumes_raw, self.container_type)
-        # TODO: Remove redundant volumes...
-        volumes = [DockerVolume.from_str(v) for v in preprocessed_volumes_list]
+        volumes = _parse_volumes(volumes_raw, self.container_type)
         volumes_from = self.destination_info.get("docker_volumes_from", docker_util.DEFAULT_VOLUMES_FROM)
 
         docker_host_props = self.docker_host_props
@@ -566,8 +576,7 @@ def containerize_command(self, command: str) -> str:
             raise Exception(f"Cannot containerize command [{working_directory}] without defined working directory.")
 
         volumes_raw = self._expand_volume_str(self.destination_info.get("singularity_volumes", "$defaults"))
-        preprocessed_volumes_list = preprocess_volumes(volumes_raw, self.container_type)
-        volumes = [DockerVolume.from_str(v) for v in preprocessed_volumes_list]
+        volumes = _parse_volumes(volumes_raw, self.container_type)
 
         run_command = singularity_util.build_singularity_run_command(
             command,