Did you ever think you picked the wrong tool for the job?
This is an open-source project, run by volunteers, with a 2-week release cycle that very frequently includes bugs and breaking changes.
It would be a mistake to run this in a “high security facilities without internet access” and “for access control and/or alarm system”.
This software is currently for home automation hobbyists with a strong stomach for breaking changes.
Home Assistant or hassio?
I was pretty specific
You could redirect all outgoing calls to the google NTP servers on your network to your own local NTP server. People do that all the time for hardcoded DNS over in the PiHole community.
I gave it a shot and installed hassio on raspbian. In fact I’m not forced anymore to use google services! This is a big plus for privacy and I have now more freedom in my setup! 
Thank’s for the hint 
I also added a RTC now to my (raspbian lite based) hassio installation and can tell it works! You just follow the tutorials for installation and home assistant grabs the time from the host system (raspbian) and this one comes from the RTC then or a configured (and available) ntp server 
Since I switched to hassio on top of raspbian I solved at least the ntp thing and I can use the DS3231 like it is expected to.
But I have no luck getting picotts (also named pico2wave) working. For various reasons I don’t like to tell big brother what to tell me…
The error message is the following:
‘pico2wave’ was not found
01:23 PM components/picotts/tts.py (ERROR)
How would it be possible for ha to reach my picotts in raspbian from it’s docker 
Mount the python file as a bind mount?
How could that work? Could you provide some information where to start? Raspbian, Hassio, Home Assistant?
It’s simple docker.
…and which file? I installed picotts and all dependencies in my raspbian system - it contains more than just one file 
and where to bind it to?..
I’m also interested getting local TTS running. Did anyone ever succeeded doing this?
No idea. As I stated before, HASSIO is a poor example of a completely offline Home Assistant install. Regular docker, that you have control over, would be a much better option, and then you can mount anything you like.
I generally agree with this and use regular HA in Docker myself but the allure of the hassio add-ons does have me wanting to switch over at times.
The problem with non-hassio is that I just don’t know how to create my own dockerfiles to run some of the apps that don’t already have a (good/working) docker image available.
As an example, I was looking at the NUT UPS add-on recently to try to figure out how to create a working docker image but it’s beyond me how the dockerfile is created by the author of the add-on and how I can make this work outside of hassio.
Yeah, I know. I already saw that. That’s what I was referring to when I said I had looked at it.
The problem is I don’t understand how to translate that dockerfile as listed there into a working image that then integrates into HA.
I’ve researched a bit on it but what I’ve found hasn’t been clear enough to me to make it “click” on how it’s done.
Any links to any good tutorials? Even better would be specifically related to that as it relates to HA.
It doesn’t need to integrate with HA.
You can use any nut ups docker container and accomplish the same thing. You’re merely connecting to an IP/host running the software. It’s over a tcp/IP connection.
Just use a premade docker image.
Get out of that mindset. It’s just another application running on the network.
I thought before the limitation was the hassOS shipped with HASSIO. Because hassOS had no way to install picoTTS. As mentioned in this thread the idea would be to install HASSIO on raspbian and install picoTTS in raspbian (exactly what I did). I don’t have a clue why it doesn’t work now… 
picotts is indeed totaly broken in hassio 
I don’t know any privacy friendly alternative, somebody does?