switch to dockerhub busybox image

[js-9]

as gcr doesn't support multiarch images (fixes #181)

[senthilrch]

@js-9 : Thanks for raising this issue. I understand the busybox image in gcr doesn't supports arm architecture. By default, kube-fledged doesn't uses the busybox image from dockerhub because the pulls are subject to ratelimiting.

I've copied the busybox:1.35.0 image from dockerhub to senthilrch/busybox:1.35.0 in dockerhub. The "senthilrch" namespace in dockerhub is exempted from ratelimiting. I've modified kube-fledged to use senthilrch/busybox:1.35.0 by default; this will fix the issue you faced with installing kube-fledged in arm cluster.