Update docker/dockerfile Docker tag to v1.15 #12363

Job	Run time
Checks and variables	2m 23s
Unit Tests	2m 30s
NJS Unit Tests	10s
Build Binary	2m 44s
Build Plus images / build	1m 10s
Build OSS images (nginx, linux/arm64, linux/amd64) / build	1m 46s
Build OSS images (ngf, linux/arm64, linux/amd64) / build	1m 37s
Helm Tests	0s
Functional tests (nginx, v1.32.3) / Run Tests	13m 21s
Functional tests (nginx, v1.25.16) / Run Tests	15m 27s
Functional tests (plus, v1.25.16) / Run Tests	14m 54s
Conformance tests (nginx, v1.25.16, false) / Run Tests	6m 49s
Conformance tests (nginx, v1.25.16, true) / Run Tests	7m 44s
Functional tests (plus, v1.32.3) / Run Tests	13m 48s
Conformance tests (nginx, v1.32.3, false) / Run Tests	6m 33s
Conformance tests (nginx, v1.32.3, true) / Run Tests	6m 47s
Conformance tests (plus, v1.25.16, false) / Run Tests	7m 2s
Package and Publish Helm Chart	0s
Conformance tests (plus, v1.25.16, true) / Run Tests	7m 2s
Conformance tests (plus, v1.32.3, false) / Run Tests	6m 27s
Conformance tests (plus, v1.32.3, true) / Run Tests	6m 46s
	2h 5m 0s

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