setcap cap_setgid+ep on the system Python binary made all Python
processes non-dumpable, blocking /proc/[pid]/fd/ access needed by
_pid_from_inode() to resolve socket inodes to PIDs.
Fix: copy the Python binary to python3-ot and setcap only the copy.
The open-terminal server uses python3-ot (has CAP_SETGID for
multi-user os.setgroups()), while user-spawned python3 stays
capability-free and dumpable.
Slim/Alpine: removed setcap entirely (multi-user mode requires sudo,
which only the full image has). Kept libcap packages installed.
README: corrected Image Variants table — multi-user mode is
full-image only.
Per Docker best practices, security patching should come from rebuilding
with updated base image tags, not from apt-get upgrade inside the
Dockerfile. Pinning to a specific patch version ensures reproducible
builds. Bump the version and rebuild to pick up new patches.