Change parent image tag in dockerfile so dependabot updates?

[snuggie12]

Kind of along the same lines as #459, would it be possible to change the tag for busybox in the dockerfile so dependabot can keep it up to date? We use stackdriver-exporter everywhere but without patch versions being released regularly we're being flagged by security about CVEs.

[eenchevlp]

CC: @SuperQ - stackdriver_exporter needs a new release desperately. I appreciate the hard work being done on it and because of that - it is widely used. However, without ongoing maintenance, it is being flagged all over the place. Even if no new features are incorporated, at least any ongoing maintenance would be of a great help for users.