It’s not limited to what you copy and paste. One of my containers has a pretty long starter script written by the container maintainer. That is needed because the application doesn’t have an official Docker version and the starter script takes care of all the necessary work arounds to get the app running inside a container. There could be something malicious in there I don’t know about, if I don’t read the whole starter script, which is probably in a language I don’t understand well.
Even more complicated : I could have been studying the starter script and made the decision it’s fine and the author trustworthy, so I pull the container image with the tag “v1. 0” every few months a new version gets released, I take a look at the changelog, if no braking changes are mentioned I pull tag v1. 1 and replace my existing container. At some point the maintainer stops mainting the container and hands over the Repository to someone else. This person unfortunately now places malicious code in the starter script and releases an update. If I would now pull that new container image I now have a rougue container.
Using Watchtower for approximately 2 years on about 20 Containers. I had 1 issue, where a container would not start after the update. The Error Message said I had an unsupported entry in the configuration file of the app. I looked up the changelog of that app, and found out that the option was removed and replaced by something else. Had to change one line in the configuration. Not really a problem for me.
Though I decided to exclude my Home Automation Container and my kasm container ( my gateway to my network, a bit like guacamole ). Those may pose problems if they are offline unexpected.