"WARNING:root:could not open file '/etc/apt/sources.list'"

We saw this: -

WARNING:root:could not open file '/etc/apt/sources.list'

during a complex series of Docker builds.

After a LOT of investigation, we concluded that it was a combination of a slightly malformed /etc/apt/sources.list file AND the use of the add-apt-repository command.

In essence, sources.list was being slightly truncated by a previous build process, meaning that the very last character of the file was NOT a CR/LF, but was some "special character".

The subsequent use of the add-apt-repository in a Dockerfile: -

add-apt-repository "deb [arch=s390x] https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable"

was completely over-writing the sources.list file with the following TWO lines: -

deb [arch=s390x] https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable

# deb-src [arch=s390x] https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable

I've not quite reproduced the problem manually, despite my best efforts with a hex editor ...

However, the TL;DR; of the situation is that malformed lines in sources.list may be the root cause of all sorts of problems .....