Provide Dockerfile target for pre-cached vm & vm-rootless #542

Job	Run time
lint (golangci-lint)	57s
build (nix-snapshotter)	1m 37s
build (image-hello)	1m 30s
build (image-redis)	1m 40s
build (image-redisWithShell)	1m 33s
integration-test (snapshotter)	4m 4s
integration-test (kubernetes)	5m 44s
integration-test (k3s)	13m 51s
integration-test (k3s-external)	15m 31s
integration-test (k3s-rootless)	13m 50s
coverage-report	39s
	1h 0m 56s

Provide feedback