I’ve got a possible networking issue here, but I can’t say with complete certainty it is network related. It happens on OS updates in Docker (network related) and building an autoloader in Composer (may be network related, to either Packagist, Bitbucket or GitHub).
Either way, I am not getting network errors in the way you are - yours is clearly network-related. Also, mine is pretty infrequent.
CircleCI are looking into it, but I’m not yet sure they have enough build machine observability to see where it is getting stuck, so my current position is that I’ll live with it.