Fix doc builder Dockerfile
# What does this PR do?

Fix the docker file in `transformers-doc-builder`.

~~(We don't need to run `doc-builder build` in the DockerFile, right? I think it is only for the CI runs.)~~

Currently, `Doc builder (Docker image build)` fails, see [this run](https://github.com/huggingface/transformers/actions/runs/2388105533).
_The documentation is not available anymore as the PR was closed or merged._<|||||>The goal of running it inside the dockerfile is to ensure it actually works before publishing the image, so that it doesn't fail at runtime. Do you know why it failed in the first place?<|||||>Thank you, I understand it better now. I will check why it fails inside docker build.<|||||>@LysandreJik 

The problem comes from the fact that `$PR_NUMBER` is not defined in the docker image build (`doc-builder`). We can use `main` instead, right?

From `--help`, I saw

```
--version VERSION     Version of the documentation to generate. Will default to the version of the package module (using `main` for a version containing dev).
```<|||||>Changed `pr_$PR_NUMBER` to `main`.<|||||>Changed to 

```
RUN doc-builder build transformers transformers/docs/source/en --build_dir doc-build-dev --notebook_dir notebooks/transformers_doc --clean --version main
```
works.

Job run page
https://github.com/huggingface/transformers/runs/6775059643?check_suite_focus=true