Fix Dockerfile
Fix Dockerfile

Use the correct busybox arch image so that docker-manifest picks up the
correct arch.

Signed-off-by: Ben Kochie <9bbf917dc63fbaa40c67921332d26cf16a50db75@gmail.com>

roman-vynar/mysqld_exporter,prometheus/mysqld_exporter,prometheus/mysqld_exporter,roman-vynar/mysqld_exporter