Act should pass proxy build args when building Docker actions

[john-tipper]

Act version

act version 0.2.76

Feature description

Act does not pass any build args to Docker when it builds Docker-based actions. As a result, if Act is run in an enterprise environment where access to the internet is through a proxy, then these actions all break and are unusable, i.e. all third-party docker actions are unusable.

Act should allow the user to optionally pass in proxy variables (HTTP_PROXY, HTTPS_PROXY and NO_PROXY) to the build of Docker actions. This should also include the lowercased versions too.