Issue: Installing datasets and transformers in a tensorflow docker image throws Permission Error on 'import transformers'

I am using a docker container, based on latest tensorflow-gpu image, to run transformers and datasets (4.0.1 and 1.1.3 respectively - Dockerfile attached below). Importing transformers throws a Permission Error to access `/.cache`:

```
$ docker run --gpus=all --rm -it -u $(id -u):$(id -g) -v $(pwd)/data:/root/data -v $(pwd):/root -v $(pwd)/models/:/root/models -v $(pwd)/saved_models/:/root/saved_models -e "HOST_HOSTNAME=$(hostname)" hf-error:latest /bin/bash

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/_/    \___//_/ /_//____/ \____//_/    /_/      /_/  \____/____/|__/


You are running this container as user with ID 1000 and group 1000,
which should map to the ID and group for your user on the Docker host. Great!

tf-docker /root > python
Python 3.6.9 (default, Oct  8 2020, 12:12:24) 
[GCC 8.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import transformers
2020-12-15 23:53:21.165827: I tensorflow/stream_executor/platform/default/dso_loader.cc:49] Successfully opened dynamic library libcudart.so.11.0
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.6/dist-packages/transformers/__init__.py", line 22, in <module>
    from .integrations import (  # isort:skip
  File "/usr/local/lib/python3.6/dist-packages/transformers/integrations.py", line 5, in <module>
    from .trainer_utils import EvaluationStrategy
  File "/usr/local/lib/python3.6/dist-packages/transformers/trainer_utils.py", line 25, in <module>
    from .file_utils import is_tf_available, is_torch_available, is_torch_tpu_available
  File "/usr/local/lib/python3.6/dist-packages/transformers/file_utils.py", line 88, in <module>
    import datasets  # noqa: F401
  File "/usr/local/lib/python3.6/dist-packages/datasets/__init__.py", line 26, in <module>
    from .arrow_dataset import Dataset, concatenate_datasets
  File "/usr/local/lib/python3.6/dist-packages/datasets/arrow_dataset.py", line 40, in <module>
    from .arrow_reader import ArrowReader
  File "/usr/local/lib/python3.6/dist-packages/datasets/arrow_reader.py", line 31, in <module>
    from .utils import cached_path, logging
  File "/usr/local/lib/python3.6/dist-packages/datasets/utils/__init__.py", line 20, in <module>
    from .download_manager import DownloadManager, GenerateMode
  File "/usr/local/lib/python3.6/dist-packages/datasets/utils/download_manager.py", line 25, in <module>
    from .file_utils import HF_DATASETS_CACHE, cached_path, get_from_cache, hash_url_to_filename
  File "/usr/local/lib/python3.6/dist-packages/datasets/utils/file_utils.py", line 118, in <module>
    os.makedirs(HF_MODULES_CACHE, exist_ok=True)
  File "/usr/lib/python3.6/os.py", line 210, in makedirs
    makedirs(head, mode, exist_ok)
  File "/usr/lib/python3.6/os.py", line 210, in makedirs
    makedirs(head, mode, exist_ok)
  File "/usr/lib/python3.6/os.py", line 220, in makedirs
    mkdir(name, mode)
PermissionError: [Errno 13] Permission denied: '/.cache'
```
I've pinned the problem to `RUN pip install datasets`, and by commenting it you can actually import transformers correctly. Another workaround I've found is creating the directory and giving permissions to it directly on the Dockerfile.

```
FROM tensorflow/tensorflow:latest-gpu-jupyter
WORKDIR /root

EXPOSE 80
EXPOSE 8888
EXPOSE 6006

ENV SHELL /bin/bash
ENV PATH="/root/.local/bin:${PATH}"

ENV CUDA_CACHE_PATH="/root/cache/cuda"
ENV CUDA_CACHE_MAXSIZE="4294967296"

ENV TFHUB_CACHE_DIR="/root/cache/tfhub"

RUN pip install --upgrade pip

RUN apt update -y && apt upgrade -y

RUN pip install transformers

#Installing datasets will throw the error, try commenting and rebuilding
RUN pip install datasets

#Another workaround is creating the directory and give permissions explicitly
#RUN mkdir /.cache
#RUN chmod 777 /.cache
```


Comments:
Thanks for reporting !
You can override the directory in which cache file are stored using for example
```
ENV HF_HOME="/root/cache/hf_cache_home"
```

This way both `transformers` and `datasets` will use this directory instead of the default `.cache`

Great, thanks. I didn't see documentation about than ENV variable, looks like an obvious solution. 

> Thanks for reporting !
> You can override the directory in which cache file are stored using for example
> 
> ```
> ENV HF_HOME="/root/cache/hf_cache_home"
> ```
> 
> This way both `transformers` and `datasets` will use this directory instead of the default `.cache`

can we disable caching directly?

Hi ! Unfortunately no since we need this directory to load datasets.
When you load a dataset, it downloads the raw data files in the cache directory inside <cache_dir>/downloads. Then it builds the dataset and saves it as arrow data inside <cache_dir>/<dataset_name>.

However you can specify the directory of your choice, and it can be a temporary directory if you want to clean everything up at one point.

I'm closing this to keep issues a bit cleaner