Add `/public/assets/img/webpack` to ignore files again (#30451)
Fixes https://github.com/go-gitea/gitea/issues/30442
It's inconvenient to have new untracked files show up in git when
switching to older branches that had generated them.
Introduce a list of such files and folders to gitignore and
dockerignore.
(cherry picked from commit 68271834d6ae6d397b5a2048f9e515ff53735994)
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