`dock_from_desc()` support for GitLab

[michkam89]

Hi,

I see that dock_from_desc() only supports CRAN and GitHub packages. I saw #16, so is it maybe planned to add GitLab as well in the near future? I would also be interested in supporting on-prem hosts here, not only gitlab.com.

If not, are you open to providing some pull requests in this matter?

Thanks!

[statnmap]

Now that we use {attachment} in {golem}, we would be able to use functionalities that detects other sources, and combine with what exists in {dockerfiler}.
Some ongoing issues are listed in {attachment} in this way: https://github.com/ThinkR-open/attachment/issues
GitLab is already accounted for in attachment::find_remotes() to fill your DESCRIPTION file.
Then, sub-functions of create_dependency_file() could be used to fill this Dockerfile as described in this issue: ThinkR-open/attachment#56