Avoid rebuilding containers unnecessarily to reduce job run time. #12

Job	Run time
build_attest_all (buildconfigs/key_xor_test_app.sh)	2m 19s
build_attest_all (buildconfigs/oak_containers_kernel.sh)	10m 21s
build_attest_all (buildconfigs/oak_containers_orchestrator.sh)	6m 19s
build_attest_all (buildconfigs/oak_containers_stage1.sh)	3m 30s
build_attest_all (buildconfigs/oak_containers_syslogd.sh)	5m 38s
build_attest_all (buildconfigs/oak_containers_system_image.sh)	12m 4s
build_attest_all (buildconfigs/oak_echo_enclave_app.sh)	2m 14s
build_attest_all (buildconfigs/oak_echo_raw_enclave_app.sh)	2m 22s
build_attest_all (buildconfigs/oak_functions_enclave_app.sh)	2m 30s
build_attest_all (buildconfigs/oak_functions_insecure_enclave_app.sh)	2m 16s
build_attest_all (buildconfigs/oak_restricted_kernel_simple_io_init_rd_wrapper_bin.sh)	2m 34s
build_attest_all (buildconfigs/stage0_bin.sh)	2m 17s
build_attest_all (buildconfigs/oak_orchestrator.sh)	2m 17s
	56m 41s

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