/etc/dropbear/run is in dropbear-run 2017.75-3build1.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 3 | #!/bin/sh
exec 2>&1
exec dropbear -d ./dropbear_dss_host_key -r ./dropbear_rsa_host_key -F -E -p 22
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