fix: unexpected exit when adduser failed in vsftpd image

[jason-cts]

Summary of Changes

run adduser only when user is not exists in vsftpd image

Why is this change being made?

I use vsftpd image by create a normal docker container,
when restart the container,
container will failed at every restart and shows adduser: user 'ftpuser' in use in docker logs,
then it just stopped itself.

this fix will run adduser in only when the container does not have that user in entrypoint.sh.

How was this tested? How can the reviewer verify your testing?

1. prepare ftp-user-password-secret file described in README.md
2. docker run --name vsftpd -d -e PASV_ADDRESS=172.17.0.1 -v ./ftp-user-password-secret:/run/secrets/ftp-user-password-secret:ro instantlinux/vsftpd
3. docker restart vsftpd

Completion checklist

• The pull request is linked to all related issues
• This change has unit test coverage
• Documentation has been updated
• Dependencies have been updated and verified