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>

Dockerfile

@@ -48,10 +48,8 @@ RUN apt-get update && \
             git-buildpackage pristine-tar sbuild schroot \
             umoci skopeo && \
     apt-get clean && \
-    rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/*
-
-RUN sed -i 's|\tGOSU="gosu builder"|\0\n\tsbuild-adduser builder >/dev/null 2>\&1|' /kas/container-entrypoint
-RUN sed -i 's|#!/bin/bash|\0\n\nupdate-binfmts --enable \&\& [ -f /proc/sys/fs/binfmt_misc/status ]|' /kas/container-entrypoint
+    rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/* && \
+    sed -i 's|# kas-isar: ||g' /kas/container-entrypoint
 
 FROM kas-base as kas
 

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