fix: use Docker socket HTTP API in swarm collector, no CLI dependency

Replace exec.CommandContext calls (docker ps, docker inspect, nc -z) with
direct HTTP calls over the Unix socket using Go's net/http + custom transport.
Also removes netcat-openbsd from Dockerfile since nc is no longer used.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Dockerfile

@@ -20,7 +20,6 @@ RUN apt-get update && apt-get install -y \
     ca-certificates \
     libvirt-clients \
     openssh-client \
-    netcat-openbsd \
     && rm -rf /var/lib/apt/lists/*
 
 WORKDIR /app