docker-container: give readiness wait a full startup timeout

[crazy-max]

relates to https://github.com/docker/buildx/actions/runs/33654939933/job/100331039441#step:7:2387

=== Failed
=== FAIL: tests TestIntegration/TestBuildPolicyImageName/worker=docker-container/config-label-allow (0.07s)
    policy_build.go:579:
        	Error Trace:	/src/tests/policy_build.go:579
        	Error:      	Received unexpected error:
        	            	exit status 1
        	Test:       	TestIntegration/TestBuildPolicyImageName/worker=docker-container/config-label-allow
        	Messages:   	ERROR: failed to build: waiting for BuildKit: DeadlineExceeded: context deadline exceeded while waiting for connections to become ready
        --- FAIL: TestIntegration/TestBuildPolicyImageName/worker=docker-container/config-label-allow (0.07s)

A recently started builder should use its StartedAt timestamp to decide whether BuildKit may still be coming up, but not as the absolute deadline for the client readiness wait.

When the first client request arrived near the end of that startup window, the wait could inherit only a few milliseconds and fail with DeadlineExceeded even though the builder was still legitimately starting. Give those recent containers a fresh bounded wait from the current time while continuing to skip the wait for established builders.