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.

[crazy-max]

Encounter this error quite often in actions-toolkit integration tests: https://github.com/docker/actions-toolkit/actions/runs/34329830393/job/102395668163?pr=1309#step:10:752

/usr/bin/docker buildx history export --builder builder-6e45ead8-b127-4744-ac90-e183cea22f82 --output /home/runner/work/_temp/docker-actions-toolkit-I68wIX/export/***~71SJ0M.dockerbuild 71sj0ml6gjf0g90srl8bmo5b4 --finalize
Writing summary
 ❯ __tests__/github/summary.test.itg.ts (7 tests | 1 failed) 12100ms
     ✓ write build summary %p  3676ms
     ✓ write build summary %p  1548ms
     ✓ write bake summary %p  1240ms
     ✓ write bake summary %p  1452ms
     × write bake summary %p 2420ms
     ✓ fails with dockerfile syntax issue  1105ms
     ✓ without build record  656ms

⎯⎯⎯⎯⎯⎯⎯ Failed Tests 1 ⎯⎯⎯⎯⎯⎯⎯

 FAIL  __tests__/github/summary.test.itg.ts > writeBuildSummary > write bake summary %p
Error: ERROR: waiting for BuildKit: DeadlineExceeded: context deadline exceeded while waiting for connections to become ready
 ❯ src/buildx/history.ts:66:15
     64|     }).then(res => {
     65|       if (res.stderr.length > 0 && res.exitCode != 0) {
     66|         throw new Error(res.stderr.trim());
       |               ^
     67|       }
     68|       return <InspectResponse>JSON.parse(res.stdout);
 ❯ History.inspect src/buildx/history.ts:61:12
 ❯ History.export src/buildx/history.ts:113:9
 ❯ __tests__/github/summary.test.itg.ts:157:23

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