Dovecot version 2.3.17-3+ubuntu20.04 and some mod

Change adduser script to create Maildir folder for user and ".forward" file correct permission

Dovecot version 2.3.16-2+ubuntu20.04

Upgrading Ubuntu from 18.04 to 20.04

From now this image is built from Ubuntu 20.04 instead of Ubuntu 18.04

Dovecot version 2.3.14-5+ubuntu18.04

2.3.14-5_r0

Update dovecot.conf

Dovecot version 2.3.14-5+ubuntu18.04

Update Dockerfile

Dovecot version 2.3.9.2-1+ubuntu18.04

Update README.md