chore(deps): update docker.io/library/alpine docker tag to v3.18.0

images/dovecot/Dockerfile

-FROM docker.io/library/alpine:3.17.3
+FROM docker.io/library/alpine:3.18.0
 
 # Update base system
 RUN apk update

images/postfix/Dockerfile

-FROM docker.io/library/alpine:3.17.3 as dhparam
+FROM docker.io/library/alpine:3.18.0 as dhparam
 
 RUN apk add --no-cache openssl
 
 RUN mkdir -p /etc/postfix
 RUN openssl dhparam -out /etc/postfix/postfix_dhparams.pem 2048
 
-FROM docker.io/library/alpine:3.17.3
+FROM docker.io/library/alpine:3.18.0
 
 # Install pre-requirements
 RUN apk add --no-cache ca-certificates rsyslog supervisor