test: adjust dkim key length

RFC6376  https://datatracker.ietf.org/doc/html/rfc6376#section-3.3.3
Specifies a key must be between 512 bits to 2048 bits.
However RSA Keys must be minimum of 1024 bits.

MR !4

include/docker-entrypoint.sh

@@ -69,7 +69,7 @@ postconf -e "myhostname = $(`echo hostname -f`)"
 
         echo "[Information] Creating DHPEM Key"
 
-        openssl dhparam -out /certs/dovecot/dh.pem 4096
+        openssl dhparam -out /certs/dovecot/dh.pem 2048
 
     fi