Make backup report hostname configurable

lib/common.sh

@@ -37,6 +37,7 @@ load_config() {
   : "${RCLONE_BWLIMIT:=0}" "${ENABLE_MAIL_REPORT:=true}" "${MAIL_TO:=johannes}"
   # Keep the old report settings; an empty sender uses app-backup@<FQDN>.
   : "${MAIL_FROM:=}" "${MAIL_SUBJECT_PREFIX:=[app-backup]}" "${MAIL_INCLUDE_LOG_TAIL_LINES:=200}"
+  : "${MAIL_HOSTNAME:=}"
   : "${MAIL_SENDMAIL_BIN:=/usr/sbin/sendmail}" "${MAIL_SEND_TIMEOUT:=30}"
   : "${ENABLE_WORDPRESS:=false}" "${ENABLE_NEXTCLOUD:=false}" "${ENABLE_NEXTCLOUD_DATA:=true}"
   : "${ENABLE_GITEA:=false}" "${ENABLE_MAIL:=false}" "${ENABLE_DB_DUMPS:=true}"
@@ -513,7 +514,7 @@ finish() {
   if [[ ${CHECK:-false} != true && $ENABLE_MAIL_REPORT == true && $OPERATION == backup ]]; then
     python3 "$LIB_DIR/report.py" deliver --sizes "$report_sizes" --log "$LOG_FILE" \
       --output "$LOG_DIR/report_$RUN_ID.eml" --to "$MAIL_TO" --sender "$MAIL_FROM" \
-      --subject-prefix "$MAIL_SUBJECT_PREFIX" --run "$RUN_ID" --exit-code "$code" \
+      --hostname="$MAIL_HOSTNAME" --subject-prefix "$MAIL_SUBJECT_PREFIX" --run "$RUN_ID" --exit-code "$code" \
       --started "$RUN_STARTED_EPOCH" --phase "$BACKUP_PHASE" --upload-status "$UPLOAD_STATUS" \
       --config "$CONFIG_FILE" --workdir "$WORKDIR" --archive-dir "$ARCHIVE_DIR" \
       --compression "$COMPRESSOR" --remote "$RCLONE_REMOTE_BASE" --sendmail "$MAIL_SENDMAIL_BIN" \