Add .dockerignore (#21753)
Add .dockerignore (#21753)

There's a lot of work that has been done on `.dockerignore`:

- #329
- #2927
- #8338

And finally, it has been deleted by #2927.

This is a copy of the `.gitignore`.
Creating a soft link is more elegant, but it may cause trouble to the Windows users.
go-gitea/gitea,go-gitea/gitea,go-gitea/gitea