Provide Dockerfile target for pre-cached vm & vm-rootless #538

Job	Run time
lint (golangci-lint)	55s
build (nix-snapshotter)	27s
build (image-hello)	22s
build (image-redis)	28s
build (image-redisWithShell)	22s
integration-test (snapshotter)	4m 9s
integration-test (kubernetes)	4m 40s
integration-test (k3s)	12m 43s
integration-test (k3s-external)	14m 29s
integration-test (k3s-rootless)	13m 2s
coverage-report	45s
	52m 22s

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