Pulling large docker containers can take longer than the default
context timeout. Without a way to change this it is very hard for
users to utilise Nomad properly without hacky work arounds.
This change adds an optional pull_timeout config parameter which
gives operators the possibility to account for increase pull times
where needed. The infra docker image also has the option to set a
custom timeout to keep consistency.
The test used old local copies of Docker structs and appeared to be
testing an outdated approach to task config decoding.
Updated to use real Docker structs so we can do end-to-end unit testing
of real Docker task configs.
