Add docker section in .drone.yml with net option

[floatdrop]

This PR introduces docker section in .drone.yml file and net option in this section.

net option should be used to configure --net parameter of Docker container.

Default value: bridge

Example:

docker:
  net: host

[bradrydzewski]

Awesome! One minor comment. We should only enable custom network settings if b.Privileged && len(b.Repo.PR) == 0. The reason is that access to the host machines networking has security implications.

[floatdrop]

@bradrydzewski fair point. I hope docker will be able work in bridge mode on IPv6-only hosts.

[floatdrop]

May be this setting should be moved to other place, because checking for b.Privileged && len(b.Repo.PR) == 0 will disable net for pull-requests.

How about moving DefaultDockerNetworkMode to drone.toml? This would solve our IPv6 problems (we will set it to host for all builds) since we are in trusted environment and not exposing drone to external network.

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On second thought - isn't it quite strange to change net option for only part of builds? Looks like huge source of bugs.