Make /bin/sh available to debug image

[anbraten]

Description

As some systems in the docker world expect the default shell to be under /bin/sh and the debug image atm copies all busybox binaries from /bin to /busybox I added a link so /bin/sh links to /busybox/sh.

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Release Notes

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Examples of user facing changes:
- debug image ships with /bin/sh

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[anbraten]

@googlebot I signed it!