Update check (#1951)

* mail binary

* initial work

* make env vars available

* typo

* some fixes

* make script ugly, to satisfy linter..

* mailserver.env updated

* Version to welcome message added

* remove VERSION file references

* VERSION --> DMS_VERSION

* fetch remote version

* variable usage

* Quoting added

* edge test & docu

* dash removed

* subject changed

* re-add VERSION

* VERSION added

* new file:   VERSION

* rewrite

* unnecessary additions from fail2ban PR removed

* UPDATE_CHECK_INTERVAL added

* syntax check & _log function

* comment added

* final commit

target/scripts/update-check.sh

@@ -0,0 +1,60 @@
+#! /bin/bash
+
+VERSION=$(</VERSION)
+VERSION_URL="https://raw.githubusercontent.com/docker-mailserver/docker-mailserver/master/VERSION"
+CHANGELOG="https://github.com/docker-mailserver/docker-mailserver/blob/master/CHANGELOG.md"
+
+function _log
+{
+  DATE=$(date '+%F %T')
+  echo "${DATE} ${1}"
+}
+
+# check for correct syntax
+# number + suffix. suffix must be 's' for seconds, 'm' for minutes, 'h' for hours or 'd' for days.
+if [[ ! ${UPDATE_CHECK_INTERVAL} =~ ^[0-9]+[smhd]{1}$ ]]
+then
+  _log "Error: Invalid UPDATE_CHECK_INTERVAL value: ${UPDATE_CHECK_INTERVAL}"
+  _log "Info: Fallback to daily update checks"
+  UPDATE_CHECK_INTERVAL="1d"
+fi
+
+while true
+do
+  # get remote version information
+  LATEST=$(curl -Lsf "${VERSION_URL}")
+
+  # did we get a valid response?
+  if [[ ${LATEST} =~ ^[0-9]+\.[0-9]+\.[0-9]+$ ]]
+  then
+    _log "Info: Remote version information fetched"
+
+    # compare versions
+    if dpkg --compare-versions "${VERSION}" lt "${LATEST}"
+    then
+      # send mail notification to postmaster
+      read -r -d '' MAIL << EOM
+Hello ${POSTMASTER_ADDRESS}!
+
+There is a docker-mailserver update available on your host: $(hostname -f)
+
+Current version: ${VERSION}
+Latest  version: ${LATEST}
+
+Changelog: ${CHANGELOG}
+EOM
+      echo "${MAIL}" | mail -s "Mailserver update available! [ ${VERSION} --> ${LATEST} ]" "${POSTMASTER_ADDRESS}" && \
+
+      _log "Info: Update available [ ${VERSION} --> ${LATEST} ]" && \
+
+      # only notify once
+      exit 0
+    else
+      _log "Info: No update available"
+    fi
+  else
+    _log "Error: Update check failed."
+  fi
+  # check again in one day
+  sleep "${UPDATE_CHECK_INTERVAL}"
+done