Adds support for configuration of the Docker client timeout via
'docker_client_timeout'.

This change also increases the default timeout to 120 seconds, as we
sometimes see timeouts in CI and heavily loaded or underpowered
environments. Increasing 'docker_client_timeout' further may be helpful
in cases where Docker reports 'Read timed out'.

Change-Id: I73745771078cb2c0ebae2b1d87ba2c4c12958d82
Closes-Bug: #1809844