Docker containers with root have rootish rights on the host machine too because the userid will just be 0 for both. So if you have, say, a bind mount that you play fast and loose with, the docker user can create 0777 files outside the docker container, and now we're almost done. Even worse if "just to make it work" someone runs the container with --privileged and then makes the terminal mistake of exposing that container to the internet.
Can you explain this a bit further? Wouldn't that 0777 file outside docker be still executed inside the container and not on the host?
I believe they meant you could create an executable that is accessible outside the container (maybe even as setuid root one), and depending on the path settings, it might be possible to get the user to run it on the host.
Imagine naming this executable "ls" or "echo" and someone having "." in their path (which is why you shouldn't): as long as you do "ls" in this directory, you've ran compromised code.
There are obviously other ways to get that executable to be run on the host, this just a simple example.
Another example is they would enumerate your directories and find the names of common scripts and then overwrite your script. Or to be even sneakier, they can append their malicious code to an existing script in your filesystem. Now each time you run your script, their code piggybacks.
OTH if I had written such a script for linux I'd be looking to grab the contents of $(hist) $(env) $(cat /etc/{group,passwd})... then enumerate /usr/bin/ /usr/local/bin/ and the XDG_{CACHE,CONFIG} dirs - some plaintext credentials are usually here.
The $HOME/.{aws,docker,claude,ssh}
Basically the attacker just needs to know their way around your OS. The script enumerating these directories is the 0777 script they were able to write from inside the root access container.
If your chosen development environment supports it, look into distroless or empty base containers, and run as --read-only if you can.
Go and Rust tend to lend themselves to these more restrictive environments a bit better than other options.