ci: sign image builds (fixing security issue) #1344

Job	Run time
changes_baseimage	4s
changes_image	7s
c / lint	9s
golang / escapes_detect	40s
developer_local / local_env (kind)	8m 4s
golang / gofmt_test	33s
golang / govet_test	1m 10s
golang / vendor	37s
golang / lint	1m 39s
golang / vulnerability_detect	31s
integration_test / build-kepler	3m 35s
unit_test / unit_test	4m 32s
yamllint / yamllint	5s
image / image_build (kepler, build/Dockerfile, linux/amd64,linux/arm64, INSTALL_DCGM=false INSTALL_HABANA...	16m 54s
image / image_build (kepler, build/Dockerfile, linux/amd64, INSTALL_DCGM=true INSTALL_HABANA=false , late...	5m 4s
image / image_build (kepler, build/Dockerfile, linux/amd64, INSTALL_DCGM=false INSTALL_HABANA=true , late...	2m 39s
image / image_build (kepler-validator, build/Dockerfile.kepler-validator, linux/amd64, latest)	2m 9s
integration_test / integration_test_with_libbpf (kind)	6m 18s
	54m 50s

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