Filter out invalid IPv6 port bindings from TestDynamicPortForward and TestMultipleDynamicPortForward

[chienhanlin]

Summary

This PR filters out invalid IPv 6 port binding to fix failed TestDynamicPortForward and TestMultipleDynamicPortForward test when Docker version >= 20.10.6.

The error log is provided as follows.

1632523820118261648 [Debug] logger=structured msg="Sending task change event" taskARN="testDynamicPortForward" status="RUNNING" sentStatus="NONE" event="testDynamicPortForward -> RUNNING, Known Sent: NONE, PullStartedAt: 0001-01-01 00:00:00 +0000 UTC, PullStoppedAt: 0001-01-01 00:00:00 +0000 UTC, ExecutionStoppedAt: 0001-01-01 00:00:00 +0000 UTC"
    engine_unix_integ_test.go:377: PortBindings was not set; should have been len 1 [{24690 49153 0.0.0.0 0} {24690 49153 :: 0}]

The related PR #3025 was merged and released in Agent version 1.55.3.

Implementation details

1. Filter out invalid IPv6 port binding when binding.BindIP == "::" as the environment variable ECS_EXCLUDE_IPV6_PORTBINDING is true by default.
2. Add comments on getValidPortBinding() function to explain why IPv6 port bindings are filtering it out in the test code.

Testing

Ran ECS_LOGLEVEL=debug /usr/local/go/bin/go test -count=1 -v -cover -race -tags integration -timeout=30m ./agent/... on an EC2 instance, launched by Amazon ECS-optimized Amazon Linux 2 AMI (amzn2-ami-ecs-hvm-2.0.20210916-x86_64-ebs), which has Docker version 20.10.7 installed.

New tests cover the changes: no

Description for the changelog

N/A

Licensing

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

[sharanyad]

discussed offline, we will add a comment to mention why we are filtering it out in the test code

[chienhanlin]

Will add comments to explain the fix in next revision. Thanks!