change ports response from Ports to KnownPortBindings

[sharanyad]

Summary

addresses #2476

Implementation details

when host port is set to 0 in task def for bridge mode, the same is used as part of http metadata response to the container.
Changing this to use the updated port bindings retrieved from docker.
Metadata file uses the port bindings from docker as well.

Testing

manual testing:
after the change

# curl $ECS_CONTAINER_METADATA_URI_V4/task | grep HostPort
                    "HostPort": 32774,

before the change

# curl $ECS_CONTAINER_METADATA_URI_V4/task | grep HostPort
#

New tests cover the changes: yes

Description for the changelog

Licensing

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[fierlion]

KnownPortBindingsUnsafe are you calling it Unsafe to guard against setting it directly as mentioned in the comments in container.go?

	// NOTE: Do not access <unsafeVar> directly. Instead, use `Get<Var>`
	// and `Set<Var>`.