Create .dockerignore to speedup build.

[hloeffler]

Add a .dockerignore file to speedup docker build by ignoring the ".git" dir.

[codecov-io]

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[sapk]

I removed the file .dockerignore specially to be able to build previous version at build stage via build variable with latest Dockerfile. https://github.com/go-gitea/gitea/blob/master/Dockerfile#L21

I personally don't think that this step need a little speed-up but I will not block if others reviewers thinks the opposite. I just give context why we don't have a .dockerignore file.

[hloeffler]

Thanks for pointing this out.
Than this PR breaks a feature, sorry for that.
I prefer to handle the versions outside the container, but i guess this is personal preferences.
Feel free to close.

[sapk]

Re-tinking of it, .git folder is needed to calculate GITEA_VERSION inside Makefile. https://github.com/go-gitea/gitea/blob/master/Makefile#L39