docker-postfix/configs/rsyslog.conf

$ModLoad immark.so # provides --MARK-- message capability
$ModLoad imuxsock.so # provides support for local system logging (e.g. via logger command)

# default permissions for all log files.
$FileOwner root
$FileGroup adm
$FileCreateMode 0640
$DirCreateMode 0755
$Umask 0022

template (name="devicelog" type="string" string="/dev/stdout")

template(name="json" type="list") {
    constant(value="{")
      constant(value="\"@timestamp\":\"")          property(name="timereported" dateFormat="rfc3339")
      constant(value="\",\"type\":\"syslog_json")
      constant(value="\",\"tag\":\"")              property(name="syslogtag" format="json")
      constant(value="\",\"relayhost\":\"")        property(name="fromhost")
      constant(value="\",\"relayip\":\"")          property(name="fromhost-ip")
      constant(value="\",\"logsource\":\"")        property(name="source")
      constant(value="\",\"hostname\":\"")         property(name="hostname" caseconversion="lower")
      constant(value="\",\"program\":\"")          property(name="programname")
      constant(value="\",\"priority\":\"")         property(name="pri")
      constant(value="\",\"severity\":\"")         property(name="syslogseverity")
      constant(value="\",\"facility\":\"")         property(name="syslogfacility")
      constant(value="\",\"severity_label\":\"")   property(name="syslogseverity-text")
      constant(value="\",\"facility_label\":\"")   property(name="syslogfacility-text")
      constant(value="\",\"message\":\"")          property(name="msg" format="json")
      constant(value="\",\"end_msg\":\"")
    constant(value="\"}\n")
}

template(name="plain" type="list") {
    property(name="timereported" dateFormat="rfc3339")
    constant(value=" ")
    property(name="syslogseverity-text" caseconversion="upper" fixedwidth="on" position.to="7")
    constant(value=" ")
    property(name="syslogtag")
    property(name="msg" spifno1stsp="on")
    property(name="msg" droplastlf="on")
    constant(value="\n")
}

if $syslogseverity <= '6' then {
    # matching logs will be saved
    action(type="omfile" DynaFile="devicelog" template="<log-format>" DirCreateMode="0755" FileCreateMode="0644")
    # enable below to stop processing further this log
    stop
}

include(file="/etc/rsyslog.d/*.conf" mode="optional")

stop