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)

.dockerignore

@@ -94,6 +94,9 @@ cpu.out
 /.air
 /.go-licenses
 
+# Files and folders that were previously generated
+/public/assets/img/webpack
+
 # Snapcraft
 snap/.snapcraft/
 parts/

.gitignore

@@ -101,6 +101,9 @@ cpu.out
 /.go-licenses
 /.cur-deadcode-out
 
+# Files and folders that were previously generated
+/public/assets/img/webpack
+
 # Snapcraft
 /gitea_a*.txt
 snap/.snapcraft/