feature request: make container startup timeout configurable

Description

canary validate fails if the container takes more than 10 seconds to start up.

That 10 second threshold is currently hard-coded.

That timeout should be configurable.

Would allow the use of canary validate with images that take longer than 10 seconds to start up, for example:

containers that do some heavy work on startup, like starting a process and polling until it passes health checks)
docker daemon that is running slowly, e.g. because it's performing I/O on a network filesystem or is in some other way resource-constrainted

it's possible to modify, via command-line argument(s), how long canary validate waits for a container to start up before timing out

Modify this

such that that 10 second timeout can be altered via configuration.

For example, I'd like to be able to run the following:

canary validate \
   --file ./checks.yaml \
   --startup-timeout 30 \
   ${IMAGE_URI}

Created this after observing this exact timeout while testing RAPIDS images over in https://github.com/rapidsai/docker/pull/670.