Document .gitignore behaviour

Description

Currently, all files that are set to be ignored in .gitignore will not be deployed by the action. This seems to be caused by the git clone operation and, while I haven’t been able to find any discussion on this in issues or PRs (I used github’s search tool, so I might be on the wrong here), I think it’s safe to assume that this is the intended behaviour.

I believe that this behaviour could be counterintuitive to some users, so it may be interesting to document it on the readme, or at least discuss the idea here.

Reproduction

1. Create a repository that includes a .gitignore file that blacklists some files
2. Set up a CI pipeline that deploys the directory that contains the .gitignore file using this action
3. Files specified on .gitignore will not be deployed

You can see this behaviour on this CI run.

Reasons why this can be unexpected

I personally got confused by this behaviour, as I expected .gitignored files to be deployed. My thought process went along the lines of:

1. People usually deploy build artifacts
2. Build artifacts are usually excluded from git through .gitignore
3. Thus, .gitignore files are probably not ignored when deploying

[corollari]

Personally, I am setting up a repository that hosts different projects, and each one of these will have it’s own CI workflow, which publishes the build artifacts to a different subdirectory on gh-pages. I was planning on doing that using keep_file on the whole repo (in a manner very similar to what you described in #324), so, at the end of the day, I will be publishing a git repository on publish_dir.

Now, it’s possible to change the CI to isolate the published subdirectories from the rest of the github repo, but that didn’t seem as something that needed to be done, so I just went ahead with that and got bit by having all of my build artifacts (which are on .gitignores) not be deployed.

With that said, I seem to be the only one that has run into this problem thus far, so I believe it’s fine to not document it, just thought that I would explain my case since other users might run into the same problem.

[corollari]

This is not a bug, sorry for botching the label.