WIP: Update github.com/fsouza/go-dockerclient and transitive dependencies

The older version of go-dockerclient has a file descriptor leak when connecting to docker over a UNIX socket (#196 (closed)). The leak is absent in latest go-dockerclient, so this MR attempts to solve the leak by upgrading it, along with its transient dependencies.

The net effect is to move from relying on runtime finalizers to close idle connections, to disabling HTTP keepalives for UNIX sockets.

An alternative approach is given in !185 (closed) - this monkey-patches go-dockerclient to fix the leak by allowing the sockets to be explicitly closed in the same way as TCP sockets. It is much lower-impact!