plugin: fix mounting /etc/hosts when running in UserNS #4

Job	Run time
validate-dco / run	20s
build	1m 50s
test (dockerd, client, integration)	16m 16s
test (dockerd, cmd/buildctl, integration)	6m 14s
test (dockerd, solver, integration)	6m 18s
test (dockerd, frontend, integration)	6m 27s
test (dockerd, frontend/dockerfile, integration)	20m 3s
test (dockerd-containerd, client, integration)	17m 24s
test (dockerd-containerd, cmd/buildctl, integration)	6m 12s
test (dockerd-containerd, solver, integration)	6m 19s
test (dockerd-containerd, frontend, integration)	6m 17s
test (dockerd-containerd, frontend/dockerfile, integration)	20m 37s
	1h 54m 17s

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