Add singularity mount options support [WIP]

[davidjurado]

Singularity read only volumes

When defining the mounts there is the need to define if these volumes are going be used with read-only permissions, to allow users to define this, here is a new definition for the inputs and outputs in the mlcube.yaml file:

opts: ro

By default singularity will use rw (read-write) permissions. Users can use the option "ro" to define a read-only volume. Singularity docs

MLCube YAML example:

tasks:
  train:
    parameters:
      inputs:
        data_dir: { type: directory, default: data/, opts: ro }
        embeddings_dir: { type: directory, default: embeddings/, opts: ro }
        input_model_dir: { type: directory, default: model/, opts: ro }
      outputs:
        output_model: { type: directory, default: output/ }

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[dfeddema]

Read only access feature for input data in containers (docker, podman, singularity) for mlcubes requested by MedPerf WG.