BuildKit: Possible runtime DoS via unbounded group parsing

BuildKit is a toolkit for converting source code to build artifacts in an efficient, expressive and repeatable manner. Prior to 0.31.1, BuildKit read attacker-controlled /etc/passwd and /etc/group files without an upper bound while resolving a username to a user identifier or group identifier in executor/oci/user.go and solver/llbsolver/ops/user_linux.go. A malicious base image or build could provide oversized files that exhausted memory during user resolution and caused out-of-memory termination of the buildkitd process. This issue is fixed in version 0.31.1.