fix(docker): add explicit env allowlist for container credentials (#1436)

Docker terminal sessions are secret-dark by default. This adds
terminal.docker_forward_env as an explicit allowlist for env vars
that may be forwarded into Docker containers.

Values resolve from the current shell first, then fall back to
~/.hermes/.env. Only variables the user explicitly lists are
forwarded — nothing is auto-exposed.

Cherry-picked from PR #1449 by @teknium1, conflict-resolved onto
current main.

Fixes #1436
Supersedes #1439

website/docs/user-guide/features/tools.md

@@ -135,6 +135,8 @@ All container backends run with security hardening:
 - Full namespace isolation
 - Persistent workspace via volumes, not writable root layer
 
+Docker can optionally receive an explicit env allowlist via `terminal.docker_forward_env`, but forwarded variables are visible to commands inside the container and should be treated as exposed to that session.
+
 ## Background Process Management
 
 Start background processes and manage them: