Support dockerfile specific ignore

[JaewonHur]

This PR adds support for Dockerfile.dockerignore (apple/container#1169).
It comes with performance overhead as it requires one more buildkit communication (i.e., ReadEntrypoint).

Test results is as below:

╰─❯ TOOL=container ./test-ignore.sh
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Running .dockerignore tests with: container
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[TEST] Test 1: Basic .dockerignore
  $ ( cd test1-basic ; container build -t test-ignore:latest . )
[PASS] included.txt was copied (expected)
[PASS] ignored.txt was NOT copied (expected)

[TEST] Test 2: Dockerfile-specific ignore (Dockerfile.dockerignore)
  $ ( cd test2-specific ; container build -t test-ignore:latest . )
[PASS] specific.txt was NOT copied (correctly ignored by Dockerfile.dockerignore)
[PASS] general.txt was copied (only ignored by .dockerignore, not Dockerfile.dockerignore)

[TEST] Test 3: Containerfile with .containerignore
  $ ( cd test3-containerfile ; container build -f Containerfile -t test-ignore:latest . )
[PASS] included.txt was copied (expected)
[FAIL] ignored.txt was copied (unexpected)

[TEST] Test 4: Containerfile-specific ignore (Containerfile.containerignore)
  $ ( cd test4-containerfile-specific ; container build -f Containerfile -t test-ignore:latest . )
[FAIL] specific.txt was copied (should be ignored by Containerfile.containerignore)
[PASS] general.txt was copied (only ignored by .containerignore, not Containerfile.containerignore)

[TEST] Test 5: Build succeeds when Dockerfile is ignored
  $ ( cd test5-ignored-dockerfile ; container build -f Dockerfile -t test-ignore:latest . )
[PASS] Build succeeded even with Dockerfile in .dockerignore
[PASS] test.txt was copied (expected)

[TEST] Test 6: Build from subdirectory context
  $ ( cd test6-subdir ; container build -f subcontext/Dockerfile -t test-ignore:latest subcontext )
[PASS] included.txt was copied (expected)
[PASS] ignored.txt was NOT copied (expected)

[TEST] Test 7: No ignore file (all files should be copied)
  $ ( cd test7-no-ignore ; container build -t test-ignore:latest . )
[PASS] file1.txt was copied (expected)
[PASS] file2.txt was copied (expected)

[TEST] Test 8: Build with --platform and Dockerfile in subdirectory
  $ ( cd test8-platform-subdir-dockerfile ; container build --platform linux/amd64 -t test-ignore:latest -f nested/project/Dockerfile . )
[FAIL] included.txt was NOT copied (should be included)
[PASS] secret.txt was NOT copied (correctly ignored by Dockerfile.dockerignore)
[FAIL] nested/project/config.txt was NOT copied (unexpected)

[TEST] Test 9: Custom-named Dockerfile (app1.Dockerfile)
  $ ( cd test9-custom-dockerfile-name ; container build -f app1.Dockerfile -t test-ignore:latest . )
[FAIL] app1-specific.txt was copied (should be ignored by app1.Dockerfile.dockerignore)
[FAIL] generic.txt was NOT copied (should be included)
[PASS] included.txt was copied (expected)

[TEST] Test 10: Custom-named Dockerfile in subdirectory
  $ ( cd test10-custom-name-subdir ; container build -f nested/project/app2.Dockerfile -t test-ignore:latest . )
[FAIL] from-app2-ignore.txt was copied (should be ignored by app2.Dockerfile.dockerignore)
[FAIL] from-root-ignore.txt was NOT copied (should be included)
[PASS] always-included.txt was copied (expected)
[PASS] nested/project/config.yaml was copied (expected)

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Test Summary
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Passed: 16
Failed: 8
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