Fix #250: bind Docker bridge local-operator trust to frontend hostname (#278)

Tightens the bridge-trust check so a connection on the Docker bridge
is only granted local-operator status when its source IP matches a
configured frontend container hostname (default: `frontend` + the
shipped `container_name` `shadowbroker-frontend`). Previously, when
`SHADOWBROKER_TRUST_DOCKER_BRIDGE_LOCAL_OPERATOR=1` was set, ANY IP
in the 172.16.0.0/12 range was granted local-operator privileges —
on a shared Docker host that included any unrelated container on the
same bridge.

Operators with renamed services can list new hostnames via the new
`SHADOWBROKER_TRUSTED_FRONTEND_HOSTS` env var (comma-separated). DNS
resolution is cached for 30s; if Docker DNS can't resolve any of the
configured names we fail closed and refuse the bridge entirely.

Single-user installs see no behavior change — the default-named
frontend container still resolves and is still trusted.

Credit: tg12 (external security audit)

docker-compose.yml

@@ -43,6 +43,11 @@ services:
       # The bundled Docker UI talks to the backend across Docker's private bridge.
       # Treat that bridge as local operator access while ports remain bound to 127.0.0.1 by default.
       - SHADOWBROKER_TRUST_DOCKER_BRIDGE_LOCAL_OPERATOR=${SHADOWBROKER_TRUST_DOCKER_BRIDGE_LOCAL_OPERATOR:-1}
+      # Issue #250: bridge trust is now bound to specific container hostnames
+      # (default: 'frontend' compose service + 'shadowbroker-frontend' container
+      # name). If you rename the frontend service or run with a different
+      # container_name, list the hostnames here (comma-separated, no spaces).
+      - SHADOWBROKER_TRUSTED_FRONTEND_HOSTS=${SHADOWBROKER_TRUSTED_FRONTEND_HOSTS:-frontend,shadowbroker-frontend}
     volumes:
       - backend_data:/app/data
     restart: unless-stopped