Update kube-addon-manager base Alpine image to 3.8.1

[awly]

Is this a BUG REPORT or FEATURE REQUEST?:

/kind feature

Alpine fixed a RCE vulnerability in apk in 3.8.1 (https://alpinelinux.org/posts/Alpine-3.8.1-released.html)
Update the base image used for addon-manager (https://github.com/kubernetes/kubernetes/blob/master/cluster/addons/addon-manager/Makefile#L22) to pick it up.

Note: I'd send a PR if it depended directly on alpine image, not bashell/alpine.

[k8s-ci-robot]

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[awly]

/assing @MrHohn

[awly]

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