Fix IPv6 Docker remote source URLs

[cuishuang]

Summary

• construct remote Docker endpoint addresses with net.JoinHostPort
• ensure IPv6 literals retain the brackets required by URL syntax
• add regression tests covering IPv4, hostname, and IPv6 Docker sources

Previously, a source such as tcp://[::1]:2375 was converted to http://::1:2375, which is not a valid URL. It is now converted to http://[::1]:2375.