This adds an Exists() interface to detect when the container is dead.
Before reporting an update error we check is Exists() is true.
Some documentation was added as well.
This canges the output of the Docker endpoint to be a map so that it is
more consistent from single to multiple returns. It also refactors
internally how we handle both types of requests.
Without this PR the /docker API endpoint is broken completely so this
change in format has no effect anyways.
These changes are tested by the upcoming integration tests.
