apt-get: unable to locate package (i.e. nano, tesseract-ocr)

In CodeEnvy terminal of the docker container, I try to install tesseract libraries via apt-get, but the package name cannot be located. It works fine in my local Ubuntu (14 / Trusty) docker container, but not on Codenvy (Ubuntu 16 / Xenial).

user@6408be1317f2:~/tomcat8$ sudo apt-get install tesseract-ocr                                                                                                                                   
Reading package lists... Done                                                                                                                                                                     
Building dependency tree                                                                                                                                                                          
Reading state information... Done                                                                                                                                                                 
E: Unable to locate package tesseract-ocr 

I also tried to install nano, but that package could also not be found.

user@6408be1317f2:~/tomcat8$ sudo apt-get install nano                                                                                                                                            
Reading package lists... Done                                                                                                                                                                     
Building dependency tree                                                                                                                                                                          
Reading state information... Done                                                                                                                                                                 
E: Unable to locate package nano

I found this on StackExchange/AskUbuntu: http://askubuntu.com/questions/378558/unable-to-locate-package-while-trying-to-install-packages-by-apt, but not sure what to make of it.

From my understanding, nano and tesseract-ocr should also be available for Ubuntu Xenial.

http://packages.ubuntu.com/xenial/nano http://packages.ubuntu.com/xenial/tesseract-ocr (this one is in the [universe] repository)

I checked /etc/apt/sources.list and all Ubuntu repos including the universe repository is enabled there.

Any idea?