Update busybox
Update busybox

Changes:

- https://github.com/docker-library/busybox/commit/b94efb4: Merge pull request https://github.com/docker-library/busybox/pull/84 from infosiftr/alpine3.12
- https://github.com/docker-library/busybox/commit/f8fa731: Update to Alpine 3.12
- https://github.com/docker-library/busybox/commit/3922e05: Merge pull request https://github.com/docker-library/busybox/pull/83 from infosiftr/buildroot-2020.05
- https://github.com/docker-library/busybox/commit/a9c111f: Update Buildroot to 2020.05

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