Enable network per build on by default

Overview

In !1569 (merged) we introduced new networking set up for the Docker executor behind the feature flag FF_NETWORK_PER_BUILD this solves a ton of problems such as #1042 (closed), #4430 (closed), #2699 (closed) and #4186 (closed) but at the moment it's something that the user has to enable.

Proposal

Set the value of FF_NETWORK_PER_BUILD to true.

Status

Progress can be tracked in gitlab-com/gl-infra&905.