ci: enable Docker NAT for six-peer regression test when available

Enable Docker NAT simulation in the six-peer regression test to provide more realistic test coverage for NAT traversal and gateway connections. This ensures that peers run behind simulated NAT routers while gateways are on the public network, matching production deployment topology. The test now checks if Docker is available before enabling Docker NAT mode, falling back to local mode if Docker is not accessible on the runner. Fixes #2204 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude <noreply@anthropic.com>

Author: sanity

.github/workflows/ci.yml

@@ -162,10 +162,18 @@ jobs:
  env:
  FREENET_CORE_PATH: ${{ github.workspace }}
  RUST_LOG: info
- run: >
- cargo test --test message_flow
- river_message_flow_over_freenet_six_peers_five_rounds
- -- --ignored --exact
+ # Use Docker NAT to simulate realistic network topology where peers
+ # are behind NAT and gateways are on the public network. This provides
+ # better test coverage for NAT traversal and gateway connection code paths.
+ # See issue #2204 for context. Only enable if Docker is accessible.
+ run: |
+ if docker info >/dev/null 2>&1; then
+ echo "Docker available - using Docker NAT backend"
+ export FREENET_TEST_DOCKER_NAT=1
+ else
+ echo "Docker not available - using local backend"
+ fi
+ cargo test --test message_flow river_message_flow_over_freenet_six_peers_five_rounds -- --ignored --exact
 
  ubertest:
  name: Ubertest