[DOCKER][MAIL] Fixed variable expansion in run

docker/mail/rootfs/etc/service/dovecot/run

@@ -1,6 +1,6 @@
 #!/bin/sh
 PID=$(pgrep dovecot)
-if [ -z ${PID} ]
+if [ -z "${PID}" ]
 then
     exec /usr/sbin/dovecot -c /etc/mail/dovecot/dovecot.conf
 fi

docker/mail/rootfs/etc/service/opendkim/run

@@ -1,6 +1,6 @@
 #!/bin/sh
 PID=$(pgrep opendkim)
-if [ -z ${PID} ]
+if [ -z "${PID}" ]
 then
     exec /usr/sbin/opendkim -x /etc/mail/opendkim/opendkim.conf
 fi

docker/mail/rootfs/etc/service/postfix/run

@@ -1,6 +1,6 @@
 #!/bin/sh
 PID=$(pgrep master)
-if [ -z ${PID} ]
+if [ -z "${PID}" ]
 then
     exec /usr/sbin/postfix start -c /etc/mail/postfix
 fi

docker/mail/rootfs/etc/service/rsyslog/run

@@ -1,6 +1,6 @@
 #!/bin/sh
 PID=$(pgrep rsyslog)
-if [ -z ${PID} ]
+if [ -z "${PID}" ]
 then
     exec /usr/sbin/rsyslogd -f /etc/mail/rsyslogd/rsyslog.conf
 fi