Thanks Elonesna! I prefer other protocol than ftp for network shares because it's more "natural/simplest" for devices that connect to them.
I finally did decide not use libreelec, it's a pity because I used to loved it in RPI!. I use it from the beginnings of openelect when I did use an AMD thinclient as HTPC, but in a modern Intel platform I feel that most of the power it's not used.
I understand that a Skylake or superior it's a waste of performance to only play multimedia files, but in my case, a complete I3 Skylake with memory and disk was cheaper than RPI 4 with the circuit shortage prices or even a good "Android TV".
Have two full performance sata (not an external clipped USB3 drive) and use it for replace part of my media+work files server did was too tempting.
Configuring lubuntu to start with a Kodi service plus complete Samba, any linux app and remote desktop to run some apllications (that be not so friendly in cli mode) was easyest than all tries in libreelec to run any file server with few users.
Libreelec it's solid as a rock, compact and friendly for the tasks that the programmers team had designed it. It's my fault to ask for something that it's not the goal of a "Just Enough" OS for Kodi. Dockers it's great but was more simplest install samba, add users and modify a conf file in seconds than take the docker way. Docker tecnology it's fantastic, but install apps was simplest, quick or most familiar almost to me. In fact, took less time than I took to write these messages. This "old" way was more solid, proven and "ad nauseum" documented.
I now have a KODI in my TV with access to all my media archive, NAS with work, ebook and photo archives with a """"complex"""" access security configuration plus a remote desktop to work in FLAC tags and file conversion and whathever I wont for the same power consumption (when it's iddle off course) of my rpi only. Mission accomplish without tears, thank to Lubuntu now and thank libreelec for many many years of excellence.