Fix panic & move to simple.go #162

Job	Run time
prepare	1s
test / prepare	5m 25s
test / run (containerd, ./frontend/dockerfile, integration)	3m 50s
test / run (containerd, ./frontend/dockerfile, dockerfile)	3m 49s
test / run (containerd-rootless, ./frontend/dockerfile, integration)	4m 3s
test / run (containerd-rootless, ./frontend/dockerfile, dockerfile)	3m 30s
test / run (containerd-1.6, ./frontend/dockerfile, integration)	4m 7s
test / run (containerd-1.6, ./frontend/dockerfile, dockerfile)	4m 22s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, integration)	4m 7s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	4m 18s
test / run (oci, ./frontend/dockerfile, integration)	3m 31s
test / run (oci, ./frontend/dockerfile, dockerfile)	3m 40s
test / run (oci-rootless, ./frontend/dockerfile, integration)	3m 17s
test / run (oci-rootless, ./frontend/dockerfile, dockerfile)	3m 49s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, integration)	3m 35s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	3m 55s
image	0s
release	0s
	59m 19s

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