Fix the /kaniko directory permissions in container

Create /kaniko directory with world permission to allow the creation of
sub directories by any user when the executor is run as non root. This
can lower the security but shouldn't have any impact in a container.

The tar unpack is the only way I found to have a directory with specific
permission as the image is created from "scratch" which doesn't have any
tool to change the permission otherwise.

Fixes GoogleContainerTools#1363

deploy/Dockerfile

@@ -50,6 +50,8 @@ RUN \
   cat /etc/ssl/certs/* > /ca-certificates.crt
 
 FROM scratch
+# Create kaniko directory with world write permission to allow non root run
+ADD files/kaniko.tar /
 COPY --from=0 /src/out/executor /kaniko/executor
 COPY --from=0 /usr/local/bin/docker-credential-gcr /kaniko/docker-credential-gcr
 COPY --from=0 /usr/local/bin/docker-credential-ecr-login /kaniko/docker-credential-ecr-login