fix(backup): mongodb backup archive file

The archive file is not a tar file, but its own thing. We shouldn't add
.tar to the path as this makes the mongorestore tool unable to recover
the file unless you rename it.

docker/backup/mongodb.sh

@@ -1,7 +1,7 @@
 #!/bin/bash
 
 mongodb() {
-    local output_file="${TMP_DIR}/${OUTPUT_FILE}.tar.gz"
+    local output_file="${TMP_DIR}/${OUTPUT_FILE}.gz"
 
     or_default "DB_PORT" "27017"