kas-isar: Rework isar-specific entrypoint setup

Leave the isar-only commands commented-out in the container-entrypoint
and simply remove the comment when building kas-isar. This is simpler,
more readable and also more robust against changes of the entrypoint
file.

While at it, avoid a separate layer for modifying container-entrypoint.

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

container-entrypoint

@@ -1,5 +1,7 @@
 #!/bin/bash
 
+# kas-isar: update-binfmts --enable && [ -f /proc/sys/fs/binfmt_misc/status ]
+
 if mount | grep -q "on / type aufs"; then
     cat <<EOF >&2
 WARNING: Generation of wic images will fail!
@@ -32,6 +34,8 @@ else
 	fi
 
 	GOSU="gosu builder"
+
+	# kas-isar: sbuild-adduser builder >/dev/null 2>&1
 fi
 
 if [ "$PWD" = / ]; then