chore(ci): add shellcheck (#7)

Reviewed-on: https://gitea.com/gitea/helm-actions/pulls/7
Reviewed-by: justusbunsi <justusbunsi@noreply.gitea.com>
Co-authored-by: Markus Pesch <markus.pesch@cryptic.systems>
Co-committed-by: Markus Pesch <markus.pesch@cryptic.systems>

.editorconfig

@@ -9,4 +9,7 @@ indent_size = 2
 end_of_line = lf
 charset = utf-8
 trim_trailing_whitespace = false
-insert_final_newline = false
+insert_final_newline = false
+
+[Makefile]
+indent_style = tab