Feature: per-socket (per-instance) exec allowlist #65

Is your feature request related to a problem?

Exec is hard-coded as denied in all three implementations: POST /containers/<name>/exec and POST /exec/<id>/start. No policy or configuration can enable it. Some deployments may legitimately want callers of one socket to run a limited set of commands in their containers, for example a health or debug command, while other sockets stay locked down.

Today's model:

• The trust boundary is the listening socket (see README "Trust model" and Policy is selected from the request body, not the caller: "per-service" enforcement has no caller identity #39). Anyone who can connect gets everything that proxy instance allows. The proxy has no caller identity.
• Policies are per service and are selected by image name only on POST /containers/create. Exec requests carry no image, so a policy cannot be selected for them.

So "per socket" means per proxy instance.

Describe the solution (for future consideration; not designed yet)

An instance-level setting, as a flag or a config file next to the policies, that allows exec under explicit limits. Open questions:

• Command allowlist, matched on Cmd in the exec-create body, using the same flag/value-pattern machinery as CmdGate. Is a plain on/off switch ever acceptable?
• Container scope: restrict to containers created from images matching a policy's allowed_image_prefixes, or to named containers. This needs the daemon's container-to-image mapping at exec time.
• Fields to force or deny: Privileged: true (always deny?), User (force it?), Env, Tty/AttachStdin.
• POST /exec/<id>/start, /resize, and GET /exec/<id>/json: start can only be allowed for exec instances this proxy created and approved, which means tracking exec IDs. Inspect leaks other users' exec command lines (verified in Routing parity: TS path-wide exec deny; Go matchEndpoint accepts endpoint subpaths #49: ExecIDs from container inspect, then GET /exec/<id>/json, shows the full arguments including inline secrets).
• Spec: a new invariant in spec/docker_socket_policy.qnt, replacing "execContainer unconditionally returns false", such as "every executed command matches the instance allowlist".
• Audit: log every exec create and start, with the command.

Describe alternatives

• Keep exec denied everywhere (status quo). Simplest and safest. Users who need exec use the daemon socket directly, outside the proxy's guarantees.
• Per-policy setting. Not workable: exec requests carry no image, so there is no policy to select (see above).
• Per-caller identity. Rejected by the Policy is selected from the request body, not the caller: "per-service" enforcement has no caller identity #39 trust model.

Which implementation(s) would this affect?

• All (Go, Rust, TypeScript, Quint specification)

Additional context

Raised while converging the exec deny rules in #49. #49 keeps exec denied and only fixes the cross-language differences.