ci: add shellcheck linter (#37682)

- Adds `make lint-shell`. uses local `shellcheck` if its version
matches, otherwise runs the pinned image via docker or podman
- `.shellcheckrc` disables the most annoying rules
- Fixes findings across existing scripts

Fixes: #37648
Signed-off-by: silverwind <me@silverwind.io>
Co-authored-by: Claude (Opus 4.7) <noreply@anthropic.com>
Co-authored-by: TheFox0x7 <thefox0x7@gmail.com>
Co-authored-by: Nicolas <bircni@icloud.com>

contrib/upgrade.sh

@@ -126,6 +126,7 @@ giteacmd manager flush-queues
 echo "Stopping gitea at $(date)"
 $service_stop
 echo "Creating backup in $giteahome"
+# shellcheck disable=SC2086 # flag string
 giteacmd dump $backupopts
 echo "Updating binary at $giteabin"
 cp -f "$giteabin" "$giteabin.bak" && mv -f "$binname" "$giteabin"