Update 'installer.sh'

installer.sh

@@ -280,6 +280,7 @@ wget --tries=3 https://git.ictmaatwerk.com/VPS-scripts/Ubuntu-Mail/raw/branch/$b
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 wget -O- https://repo.dovecot.org/DOVECOT-REPO-GPG | sudo apt-key add -
 echo "deb https://repo.dovecot.org/ce-2.3-latest/ubuntu/$(lsb_release -cs) $(lsb_release -cs) main" | sudo tee -a /etc/apt/sources.list.d/dovecot.list
+apt update
 apt install dovecot-imapd dovecot-lmtpd dovecot-pop3d dovecot-mysql -y
 
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