proftpd: restart loop on docker-compose due to adduser error

[richardg867]

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I run instantlinux/proftpd as part of a docker-compose stack, with restart set to always. Today, I found the server was no longer running, and docker-compose did not restart it as expected. Instead, the container's log had a flood of:

adduser: user 'backup' in use

...where backup is my configured FTPUSER_NAME. The entrypoint script should not attempt to create the FTP user if it somehow already exists.

[instantlinux]

Figured out how to reproduce, posting a fix, thanks for the report!