gitignore: slight changes so that it might be reused as dockerignore
gitignore: slight changes so that it might be reused as dockerignore

This diff should not change what is matched by gitignore
(e.g. `build/` matches `contrib/build-wine/build/`).
I am experimenting with reusing gitignore as dockerignore which uses
slightly different semantics.

related: https://github.com/spesmilo/electrum/pull/7431#discussion_r677541811

wakiyamap/electrum-mona,wakiyamap/electrum-mona,wakiyamap/electrum-mona,spesmilo/electrum,spesmilo/electrum,spesmilo/electrum,pooler/electrum-ltc,pooler/electrum-ltc,pooler/electrum-ltc,pooler/electrum-ltc,spesmilo/electrum,wakiyamap/electrum-mona