no "apt-get update" / Internet connection in Docker ubuntu Image

Hey folks!
I'm running a ubuntu 18.04 in acontainer with the following parameters:

Container Name: ubuntu
Sync time with Host: enabled
Restart: always
Network mode: Bridge
Host name: ubuntu
Extra arguments: -it

other Parameters are empty or default.

The Container is running and when i connect via "docker exec -i -t ubuntu /bin/bash" i'm able to work in the container.
Fist i want to update and upgrade the package sources to install some programms, but i get the following error:

root@ubuntu:/# apt-get update
Err:1 http://archive.ubuntu.com/ubuntu bionic InRelease
  Temporary failure resolving 'archive.ubuntu.com'
Err:2 http://security.ubuntu.com/ubuntu bionic-security InRelease
  Temporary failure resolving 'security.ubuntu.com'
Err:3 http://archive.ubuntu.com/ubuntu bionic-updates InRelease
  Temporary failure resolving 'archive.ubuntu.com'
Err:4 http://archive.ubuntu.com/ubuntu bionic-backports InRelease
  Temporary failure resolving 'archive.ubuntu.com'
Reading package lists... Done
W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/bionic/InRelease  Temporary failure resolving 'archive.ubuntu.com'
W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/bionic-updates/InRelease  Temporary failure resolving 'archive.ubuntu.com'
W: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/bionic-backports/InRelease  Temporary failure resolving 'archive.ubuntu.com'
W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/bionic-security/InRelease  Temporary failure resolving 'security.ubuntu.com'
W: Some index files failed to download. They have been ignored, or old ones used instead.
root@ubuntu:/# cat /etc/resolv.conf 
# Dynamic resolv.conf(5) file for glibc resolver(3) generated by resolvconf(8)
#     DO NOT EDIT THIS FILE BY HAND -- YOUR CHANGES WILL BE OVERWRITTEN
nameserver 192.168.26.254

The DNS is correct an i already tryed to change it to 8.8.8.8 in the OMV configuration of my networkinterface, without success.
I also attempt "Host" or "macvlan" as Network mode with the same result (on "macvlan" i can ping the container from my network).

My other containers like jdownloader or duplicati are working either in bridged or host network mode with working internet connection.

What could be the reason that i can not update or install any packages from the internet?no "apt-get update" / Internet connection in Docker ubuntu Image?

Okay so maybe thats my problem, beause i want to change my ubuntu vm environment into an docker container.
So it's not possible to have more than one application running in an docker container?
I thought that there are "preinstalled / preconfigured" images and if i use a "naked" ubuntu image i can install my services and applications as i want.

So is it not possible to have a ubuntu container running with duckdns, jdownloader and pihole for example?
In my case i need openHAB to control my house, but with an additional part of software called "socat" (see the link for details)

https://community.openhab.org/…ser2net-socat-guide/34895

I was running this setup in a VM, now i thought i could change my VM setup into an ubuntu docker container...
Other services linke jdownloader, duplicati, zoneminder, kodi are running in seperate containers without any problems.
I also know that there is a openhab docker image available, but i thought its impossible to install additional software into "prebuild" images, so i was trying the ubuntu image.

Anyway for the moment i will setup my old VM again and have my openHAB and socat running there.

I need socat on the same instance as my openhab. Socat creates a serial USB device which i need to forward to the openhab container if i use two seperate containers...

I have to share a virtual device between the containers no volumes or paths