Funny, when I open the link hass.io I get the following presented directly:
The advantages of using Hass.io:
So it call’s itself FOSS but includes proprietary google services? And 100% local home automation is not possible if I follow this thread? 
The headline is “The advantages of using Hass.io”
It doesn’t include any proprietary services at all. HassOS uses Google’s NTP servers for time sync. Yes, the developer could have made that configurable, but you could always open an issue.
So submit a PR with that feature.
I’m not sure how you got to there, the use of Google’s NTP servers is not a privacy concern. Yes, the default Google TTS could be viewed that way, but if you’ve run into problems with the offline TTS components, open an issue.
You are free to fork the open source and compile your own version. You have the freedom (liberty) to fork & modify the software. It happens to include some free-to-use (no cost) services with which you disagree. Apparently you are confised about the definition of FOSS.
You are NOT free to complain and have the developers cater to your desires unless you hire them to explicitly do so.
The modern definition defines free software by whether or not the recipient has the following four freedoms:[8]
- The freedom to run the program as you wish, for any purpose (freedom 0).
- The freedom to study how the program works, and change it so it does your computing as you wish (freedom 1). Access to the source code is a precondition for this.
- The freedom to redistribute copies so you can help your neighbor (freedom 2).
- The freedom to distribute copies of your modified versions to others (freedom 3). By doing this you can give the whole community a chance to benefit from your changes. Access to the source code is a precondition for this.
Freedoms 1 and 3 require source code to be available because studying and modifying software without its source code is highly impractical.
It can be OFFLINE if you configure it yourself correctly. It cannot be OFFLINE as released by Hassio though.
I don’t think anyone has posted the the ntp servers can be changed and are not hard coded. (since November) You could change them before but it was difficult.
https://github.com/home-assistant/hassos/blob/dev/Documentation/configuration.md (see timesyncd.conf)
There is also a new chrony addon in the community repo that can be used.
Also, I don’t think there’s anything proprietary about ntp servers. I may be wrong.
I remember people a while back having issues with getting time over NTP, that’s great that the NTP is not hardcoded anymore, I should have checked the docs before posting.
If you run a local NTP server then HassIO looks like you should in theory be able to run local only and offline.
I use Home Assistsnt without internet access and have no problem using the position of the sun in my automations. It’s a calculation, not an internet service.
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?
You have no idea if this setup works - right?