fix: handle HTTPS Docker endpoints without downgrading to HTTP

Docker containers widget silently downgrades HTTPS to HTTP because:
1. The prefix check only matches tcp:// and http://, so https:// falls
   through to the Unix socket code path
2. The request URL hardcodes 'http://' prefix

This patch adds https:// to the prefix check, preserves the parsed URL
scheme, and defaults the port to 443 for HTTPS connections.

Fixes #992