kas-container: Add support for checkout and for-all-repos

Allow invoking those two new sub-commands also via the container
interface.

Closes #51
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

container-entrypoint

@@ -40,7 +40,7 @@ fi
 
 if [ -n "$1" ]; then
 	case "$1" in
-	build|shell|-*)
+	build|checkout|for-all-repos|shell|-*)
 		exec $GOSU kas "$@"
 		;;
 	*)