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.

.dockerignore

@@ -95,6 +95,9 @@ cpu.out
 /.air
 /.go-licenses
 
+# Files and folders that were previously generated
+/public/assets/img/webpack
+
 # Snapcraft
 snap/.snapcraft/
 parts/